There is a new way of customizing your styling in Bootstrap 4. You can import the particular Bootstrap component you would want to modify. I normally find myself modifying colors in most of my Rails Projects
.
@import '/bootstrap/_variables.scss' // The source of my Bootstrap
// Colour Scheme
$primary: #1a284d // My primary colour
$secondary: #1eff23 // My Secondary colour
Then I allow the bootstrap gem and sass-rail gem to make the compilation for me.
`Gem file`
...
# Bootstrap Gem for my HTML framework
gem 'bootstrap', '~> 4.1.1'
# Use SCSS for stylesheets
gem 'sass-rails', '~> 5.0'
...