I've not done this myself, but this page in the authoring tool wiki has instructions on modifying the Vanilla theme for use with the authoring tool: https://github.com/adaptlearning/adapt_authoring/wiki/Modifying-the-Vanilla-Theme

You need to duplicate the vanilla theme, modify it, zip the files up then upload it in the authoring tool, then you can select the new theme for your course.

I'm not sure if there's a way to get real time changes in your preview by editing files, maybe you could use Chrome dev tools to tweak the css then apply those changes to your theme before zipping and uploading as a workaround?