If you create app/assets/stylesheets/custom.scss, the default css will be replaced by the content in custom.scss.
If you want to customize for example the vibrant color of your mastodon instance, you can put the following code in your
custom.scss file :
$color4: #d3d900; // vibrant
@import 'application';Don't forget to recompile your assets and restart mastodon(if you didn't have a custom.scss file before)
to see the changes.