Overview Focus Ring Theming Issue in Niri

### Feature Description

Thank you very much for creating DMS. I use it both on my Niri and Hyprland installations, and in both cases they turn both compositors into a complete desktop environment for those who don't have the time to rice their environments the old fashioned way.

With this being said, please spend a little more time on Niri's overview mode -- specifically on the focus rings/ borders so they may match the rest of the color theming on the desktop., or provide more options where customizing overview mode is possible.

Once again, I thank you for your phenomenal work and have an excellent evening.

### Use Case

_No response_

### Compositor(s)

- [ ] All compositors
- [x] Niri
- [ ] Hyprland
- [ ] MangoWC (dwl)
- [ ] Sway
- [ ] Other (specify below)

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

The issue concerns Niri's overview mode and its focus rings or borders, but names no file, test, or entry point. Start by locating the overview implementation and existing compositor-specific theming options. Done would require a defined way for the focus ring styling to match the desktop theme or be customized, which the issue does not specify.

Written by the indexing model from the issue text.