I am working on a multi select input control from {shinyWidgets}, but the default font color makes it difficult to read.

Here is an example:

library(shiny)
library(shinyWidgets)
  
ui <- fluidPage(
  multiInput(inputId = "id", label = "Datasets:", choices = 1:5), 
  verbatimTextOutput(outputId = "res")
)

server <- function(input, output, session) {
  output$res <- renderPrint({ input$id })
}

shinyApp(ui = ui, server = server)

How can I change the font color?

You can style a multiselect input control (i.e., shinyWidgets::multiInput()) as desired with CSS.

To get some ideas about how to do this here is the relevant CSS file from the {shinyWidgets} github page, but because that file is minified and therefore difficult to read, you might instead look at the CSS file here.

Contents of www/shinyWidgets_multiInput.css

.multi-wrapper .non-selected-wrapper {
  background: black;
}
.multi-wrapper .selected-wrapper { 
  background: black; 
}
.multi-wrapper .item {
  color: white;
}
.multi-wrapper .non-selected-wrapper .item.selected {
  opacity: .5;
  color: white;
}
.multi-wrapper .selected-wrapper .item {
  color: white;
}

Contents of app.R

library(shinyWidgets)

ui <- fluidPage(
  tags$head(
    tags$link(
      rel = "stylesheet", 
      type = "text/css", 
      href = "shinyWidgets_multiInput.css"
    )
  ),
  multiInput(
    inputId = "id",
    label = "Fruits :",
    choices = c(
      "Banana",
      "Blueberry",
      "Cherry",
      "Coconut",
      "Grapefruit",
      "Kiwi",
      "Lemon",
      "Lime",
      "Mango",
      "Orange",
      "Papaya"
    ),
    selected = "Banana",
    width = "350px"
  ), verbatimTextOutput(outputId = "res")
)

server <- function(input, output, session) {
  output$res <- renderPrint({
    input$id
  })
}

shinyApp(ui = ui, server = server)

Output

_{Reprex files hosted with on GitHub}