best way to handle CSS is trough SASS (SCSS). Default theme's SASS files are located in AppThemes/Default folder, and instructions for using it you can find here
. Also, you can edit CSS file which is also located in AppThemes/Default folder, but it's not recommended to go that way. All styling considering particular module you can find by searching SCSS files for certain CSS class you wish to edit.
I hope this will help.