Make Cycle Layouts popup follow gnome-shell theme

**Current behaviour**
I am using GNOMEs light shell theme (set with `gsettings set org.gnome.desktop.interface color-scheme "prefer-light"` or the `Light-Style` extension).

The "cycle layouts" popup window does not follow the light scheme. This is the dialog that shows when using the `Cycle layouts` keybinding:

The popup window that appears when dragging a window to the top panel, does however appear in the expected light colors:

Problems:
- An obvious inconsistency.
- I bigger problem: It is hard to distinguish the selected layout from the other layouts.

**Describe the feature you'd like**
For the cycle layouts popup to follow the chosen shell theme instead of hard coded colors, in the same way that it is already implemented for other popups (as described above).

**Additional context**
Some might consider this a bug. I would rather say that the current extension is optimized for the "default" shell theme, and better support for the light shell theme is a new feature.

Thank you for all the amazing work you do on this project!