On the client side you use the TUI-framework (totara user interface) which depends on a build process for changes to take effect. Please find further information here: https://help.totaralearning.com/display/DEV/Tui+front-end+framework

I'm not sure you can override variables on the server side, but if you want to try it.

Create a custom.css in /server/theme/mytheme/style/ and include the css in your theme config ../theme/mytheme/config.php with

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