Customize the body with CSS

You can customize the CSS by using the tags() function, for example you can add css to headers in the body in the following manner.

body <- dashboardBody(
    tags$head(
        tags$style(
            HTML('
            \\Add CSS here
            ')
        )
    )
)

Add the following CSS to make the box title bold by updating the font in the h3 header in the body:

h3 {
    font-weight: bold;
}

Using a combination of both code blocks provided above, update the CSS to make the box titles bold.
Rerun the shiny app with these updates.