Design Shiny UI

Responsive, accessible Shiny UI. bslib theming, modern layout primitives, custom CSS.

When Use

• Build new Shiny app UI from scratch
• Modernize existing Shiny app: fluidPage → bslib
• Apply brand theming (colors, fonts) to Shiny app
• Make Shiny app responsive across screen sizes
• Improve accessibility

Inputs

• Required: App purpose + target audience
• Required: Layout type (sidebar, navbar, fillable, dashboard)
• Optional: Brand colors + fonts
• Optional: Use custom CSS/SCSS? (default: bslib only)
• Optional: Accessibility needs (WCAG level)

Steps

Step 1: Pick Page Layout

bslib has 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"))
  )
)

Got: Page layout matches app's navigation + content needs.

If fail: Layout wrong? Check you use page_sidebar() / page_navbar() (bslib), not fluidPage() / navbarPage() (base shiny). bslib has better defaults + theming.

Step 2: Config 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 interactive theme editor during dev:

bslib::bs_theme_preview(my_theme)

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

If fail: Fonts don't load? Check internet (Google Fonts needs it) or switch to system fonts: font_collection("system-ui", "-apple-system", "Segoe UI"). Theme vars don't apply? Check theme is passed to page func.

Step 3: Build Layout with Cards + 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, optional header/footer
• value_box() — KPI display, icon + theme
• layout_sidebar() — nested sidebar inside cards
• navset_card_tab() — tabbed cards

Got: Responsive grid adapts to screen size.

If fail: Columns stack odd on wide screens? col_widths sum must = 12 (Bootstrap grid). Cards overlap? Set fill = FALSE on non-filling rows.

Step 4: Add Dynamic UI

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)
  # )
}

Got: UI updates dynamically from user selections + data.

If fail: Dynamic UI flickers? Use conditionalPanel() (CSS-based) not renderUI() where possible. Dynamic inputs lose values on re-render? Add session$sendInputMessage() to restore state.

Step 5: Custom CSS/SCSS (Optional)

For styles beyond bslib theme vars:

# 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")),
  # ...
)

SCSS integration with bslib:

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

Got: Custom styles applied. bslib theming not broken.

If fail: Custom CSS conflicts with bslib? Use Bootstrap CSS vars (var(--bs-primary)) not hardcoded colors. Theme changes then propagate.

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...
    )
  )
)

Got: App meets WCAG 2.1 AA: color contrast, keyboard nav, screen reader compat.

If fail: Test with browser dev tools accessibility audit (Lighthouse). Check color contrast with WebAIM's checker. All interactive elements keyboard-focusable.

Checks

• Page layout renders right on desktop + mobile widths
• bslib theme applies consistently
• Value boxes display right themes + icons
• Cards resize in responsive grid
• Custom CSS uses Bootstrap vars, not hardcoded values
• All plots have alt text
• Color contrast ≥ WCAG AA (4.5:1 for text)
• Interactive elements keyboard accessible

Pitfalls

• Mixing old + new Shiny UI: Don't mix fluidPage() with bslib components. Use page_sidebar(), page_navbar(), or page_fillable() only.
• Hardcoded colors in CSS: Use var(--bs-primary) not #2c3e50. Hardcoded colors break when theme changes.
• Missing fill = FALSE on non-filling rows: Value box + summary rows usually shouldn't stretch. Set fill = FALSE.
• Google Fonts in offline env: App deploys to air-gapped network? Use system fonts or self-hosted font files, not font_google().
• Ignoring mobile: Test with browser responsive mode. layout_columns auto-stacks on narrow screens, custom CSS may not.

See Also

• scaffold-shiny-app — initial app setup incl theme config
• build-shiny-module — modular UI components
• optimize-shiny-performance — perf-conscious rendering
• review-web-design — visual design review: layout, typography, colour
• review-ux-ui — usability + accessibility review