Customizing the Admin (backend) Panel

To customize the Admin panel with your own custom css you will need to create this file located in your current Rails app.

app/assets/stylesheets/spree/backend/globals/_variables_override.scss

You can find the variables that you can modify here

You can also add a custom color scheme already included with backend by adding this to your _variables_override.scss

@import 'spree/backend/themes/blue_steel/globals/_variables_override';

Update 5/8/2019

My setup may be different from yours, but I had to add all this to vendor/assets/stylesheets/spree/backend/globals/_variables_override.scss

Because currently my admin receives its styles from vendor/assets/stylesheets/spree/backend/all.css

/*
 * This is a manifest file that'll automatically include all the stylesheets available in this directory
 * and any sub-directories. You're free to add application-wide styles to this file and they'll appear at
 * the top of the compiled file, but it's generally better to create a new file per style scope.
 *
 *= require spree/backend
 *= require_self
 *= require_tree .
 *= require spree/backend/solidus_trackers
*/

Also had to add this to config/application.rb for it to work on Heroku

config.assets.paths << Rails.root.join("vendor", "assets", "stylesheets", "spree", "backend")