Design Shiny UI

Design responsive, accessible Shiny application interfaces using bslib theming, modern layout primitives, and custom CSS.

When to Use

• Building a new Shiny app UI from scratch
• Modernizing an existing Shiny app from fluidPage to bslib
• Applying brand theming (colors, fonts) to a Shiny app
• Making a Shiny app responsive across screen sizes
• Improving accessibility of a Shiny application

Inputs

• Required: Application purpose and target audience
• Required: Layout type (sidebar, navbar, fillable, dashboard)
• Optional: Brand colors and fonts
• Optional: Whether to use custom CSS/SCSS (default: bslib only)
• Optional: Accessibility requirements (WCAG level)

Procedure

Step 1: Choose the Page Layout

bslib provides several page constructors:

# Sidebar layout — most common for data apps
ui <- page_sidebar(
  title = "My App",
  sidebar = sidebar("Controls here"),
  "Main content here"
)

# Navbar layout — for multi-page apps
ui <- page_navbar(
  title = "My App",
  nav_panel("Tab 1", "Content 1"),
  nav_panel("Tab 2", "Content 2"),
  nav_spacer(),
  nav_item(actionButton("help", "Help"))
)

# Fillable layout — content fills available space
ui <- page_fillable(
  card(
    full_screen = TRUE,
    plotOutput("plot")
  )
)

# Dashboard layout — grid of value boxes and cards
ui <- page_sidebar(
  title = "Dashboard",
  sidebar = sidebar(open = "closed", "Filters"),
  layout_columns(
    fill = FALSE,
    value_box("Revenue", "$1.2M", theme = "primary"),
    value_box("Users", "4,521", theme = "success"),
    value_box("Uptime", "99.9%", theme = "info")
  ),
  layout_columns(
    card(plotOutput("chart1")),
    card(plotOutput("chart2"))
  )
)

Expected: Page layout matches the application's navigation and content needs.

On failure: If the layout doesn't look right, check that you're using

page_sidebar()

/

page_navbar()

(bslib) not

fluidPage()

/

navbarPage()

(base shiny). The bslib versions have better defaults and theming support.

Step 2: Configure the bslib Theme

my_theme <- bslib::bs_theme(
  version = 5,                      # Bootstrap 5
  bootswatch = "flatly",            # Optional preset theme
  bg = "#ffffff",                   # Background color
  fg = "#2c3e50",                   # Foreground (text) color
  primary = "#2c3e50",              # Primary brand color
  secondary = "#95a5a6",            # Secondary color
  success = "#18bc9c",
  info = "#3498db",
  warning = "#f39c12",
  danger = "#e74c3c",
  base_font = bslib::font_google("Source Sans Pro"),
  heading_font = bslib::font_google("Source Sans Pro", wght = 600),
  code_font = bslib::font_google("Fira Code"),
  "navbar-bg" = "#2c3e50"
)

ui <- page_sidebar(
  theme = my_theme,
  title = "Themed App",
  # ...
)

Use the interactive theme editor during development:

bslib::bs_theme_preview(my_theme)

Expected: App renders with consistent brand colors, fonts, and Bootstrap 5 components.

On failure: If fonts don't load, check internet access (Google Fonts requires it) or switch to system fonts:

font_collection("system-ui", "-apple-system", "Segoe UI")

. If theme variables don't apply, check that you're passing

theme

to the page function.

Step 3: Build the Layout with Cards and Columns

ui <- page_sidebar(
  theme = my_theme,
  title = "Analysis Dashboard",
  sidebar = sidebar(
    width = 300,
    title = "Filters",
    selectInput("dataset", "Dataset", choices = c("iris", "mtcars")),
    sliderInput("sample", "Sample %", 10, 100, 100, step = 10),
    hr(),
    actionButton("refresh", "Refresh", class = "btn-primary w-100")
  ),

  # KPI row — non-filling
  layout_columns(
    fill = FALSE,
    col_widths = c(4, 4, 4),
    value_box(
      title = "Observations",
      value = textOutput("n_obs"),
      showcase = bsicons::bs_icon("table"),
      theme = "primary"
    ),
    value_box(
      title = "Variables",
      value = textOutput("n_vars"),
      showcase = bsicons::bs_icon("columns-gap"),
      theme = "info"
    ),
    value_box(
      title = "Missing",
      value = textOutput("n_missing"),
      showcase = bsicons::bs_icon("exclamation-triangle"),
      theme = "warning"
    )
  ),

  # Main content row
  layout_columns(
    col_widths = c(8, 4),
    card(
      card_header("Distribution"),
      full_screen = TRUE,
      plotOutput("main_plot")
    ),
    card(
      card_header("Summary"),
      tableOutput("summary_table")
    )
  )
)

Key layout primitives:

• layout_columns()

  — responsive grid with

  col_widths

• card()

  — content container with optional header/footer

• value_box()

  — KPI display with icon and theme

• layout_sidebar()

  — nested sidebar within cards

• navset_card_tab()

  — tabbed cards

Expected: Responsive grid layout that adapts to screen size.

On failure: If columns stack unexpectedly on wide screens, check

col_widths

sum equals 12 (Bootstrap grid). If cards overlap, ensure

fill = FALSE

on non-filling rows.

Step 4: Add Dynamic UI Elements

server <- function(input, output, session) {
  output$dynamic_filters <- renderUI({
    data <- current_data()
    tagList(
      selectInput("col", "Column", choices = names(data)),
      if (is.numeric(data[[input$col]])) {
        sliderInput("range", "Range",
          min = min(data[[input$col]], na.rm = TRUE),
          max = max(data[[input$col]], na.rm = TRUE),
          value = range(data[[input$col]], na.rm = TRUE)
        )
      } else {
        selectInput("values", "Values",
          choices = unique(data[[input$col]]),
          multiple = TRUE
        )
      }
    )
  })

  # Conditional panels (no server round-trip)
  # In UI:
  # conditionalPanel(
  #   condition = "input.show_advanced == true",
  #   numericInput("alpha", "Alpha", 0.05)
  # )
}

Expected: UI elements update dynamically based on user selections and data.

On failure: If dynamic UI flickers, use

conditionalPanel()

(CSS-based) instead of

renderUI()

where possible. If dynamic inputs lose their values on re-render, add

session$sendInputMessage()

to restore state.

Step 5: Add Custom CSS/SCSS (Optional)

For styles beyond bslib theme variables:

# Inline CSS
ui <- page_sidebar(
  theme = my_theme,
  tags$head(tags$style(HTML("
    .sidebar { border-right: 2px solid var(--bs-primary); }
    .card-header { font-weight: 600; }
    .value-box .value { font-size: 2.5rem; }
  "))),
  # ...
)

# External CSS file (place in www/ directory)
ui <- page_sidebar(
  theme = my_theme,
  tags$head(tags$link(rel = "stylesheet", href = "custom.css")),
  # ...
)

For SCSS integration with bslib:

my_theme <- bslib::bs_theme(version = 5) |>
  bslib::bs_add_rules(sass::sass_file("www/custom.scss"))

Expected: Custom styles applied without breaking bslib theming.

On failure: If custom CSS conflicts with bslib, use Bootstrap CSS variables (

var(--bs-primary)

) instead of hardcoded colors. This ensures theme changes propagate to custom styles.

Step 6: Ensure Accessibility

# Add ARIA labels to inputs
selectInput("category", "Category",
  choices = c("A", "B", "C")
) |> tagAppendAttributes(`aria-describedby` = "category-help")

# Add alt text to plots
output$plot <- renderPlot({
  plot(data(), main = "Distribution of Values")
}, alt = "Histogram showing the distribution of selected values")

# Ensure sufficient color contrast in theme
my_theme <- bslib::bs_theme(
  version = 5,
  bg = "#ffffff",      # White background
  fg = "#212529"       # Dark text — 15.4:1 contrast ratio
)

# Use semantic HTML
tags$main(
  role = "main",
  tags$h1("Dashboard"),
  tags$section(
    `aria-label` = "Key Performance Indicators",
    layout_columns(
      # value boxes...
    )
  )
)

Expected: App meets WCAG 2.1 AA standards for color contrast, keyboard navigation, and screen reader compatibility.

On failure: Test with browser dev tools accessibility audit (Lighthouse). Check color contrast ratios with WebAIM's contrast checker. Ensure all interactive elements are keyboard-focusable.

Validation

• Page layout renders correctly on desktop and mobile widths
• bslib theme applies consistently to all components
• Value boxes display with correct themes and icons
• Cards resize properly in the responsive grid
• Custom CSS uses Bootstrap variables, not hardcoded values
• All plots have alt text for screen readers
• Color contrast meets WCAG AA (4.5:1 for text)
• Interactive elements are keyboard accessible

Common Pitfalls

• Mixing old and new Shiny UI: Don't mix

  fluidPage()

  with bslib components. Use

  page_sidebar()

  ,

  page_navbar()

  , or

  page_fillable()

  exclusively.
• Hardcoded colors in CSS: Use

  var(--bs-primary)

  instead of

  #2c3e50

  . Hardcoded colors break when the theme changes.
• Missing

  fill = FALSE

  on non-filling rows: Value box rows and summary rows usually shouldn't stretch to fill available space. Set

  fill = FALSE

  .
• Google Fonts in offline environments: If the app deploys to an air-gapped network, use system fonts or self-hosted font files instead of

  font_google()

  .
• Ignoring mobile: Test with the browser responsive mode.

  layout_columns

  automatically stacks on narrow screens, but custom CSS may not.

Related Skills

• scaffold-shiny-app

  — initial app setup including theme configuration

• build-shiny-module

  — create modular UI components

• optimize-shiny-performance

  — performance-conscious rendering

• review-web-design

  — visual design review for layout, typography, and colour

• review-ux-ui

  — usability and accessibility review