I have the latest version of Figma running on my Intel Macbook OS14.6.1.
I created a brand new Slide Deck “from scratch,” (which actually uses the “Light Slides” template by Figma) and, when Design mode is active, nothing happens if I click on the “New style or variable” plus icon button.
I am also not able to change the names of color variables, or modify the existing color variables. I can only add Theme/Template (inconsistent naming) colors when NOT in design mode. I guessing this is a limitation of the current Beta version.
So for those having similar issues, the only way to change “variables,” which are actually Theme/Template colors, is to turn off design mode and adjust the colors there, but these colors cannot be named.
Furthermore, the only way to modify a Theme/Template color (the only way to access the tiny circular color modification icon button) is to have an object with that color selected, which results in changing the color of whatever object is selected just to modify the theme colors.