Taken from https://jira.unity3d.com/browse/UIT-1494:

Summary:

I have Palette.uss file with a :root selector and some variable definitions.

My Theme file references this USS.

In the UI Builder, with the correct Theme set, trying to Set Varible on a style property shows "List is empty".

Steps:
You can experience this by extracting the attach package in a project. Make sure to select "Theme". For example modify the background-color in the "action-button" selector.

Extra Notes:

Adding the Palette.uss to the current document directly did not solve the issue. According to Alexandre Dzimi Mve it might be considered an improvement to support this.

The only workaround I found is to directly add the :root selector with the variables to the main GameScreen.uss file. Not sure how it's different than Palette.uss.

Expected Results:
The variables defined in Palette should be suggested

Actual Results:
The variables defined in Palette are not suggested

Tested on:
Unity Version 2021.2.0b10, Mac
changeset: 528997:2e1b2da87b7b
branch: 2021.2/staging
date: Tue Aug 24 09:29:24 2021 +0000