Inverse theme for notifications

### Feature Description

I'd like a way to set the notifications to light theme while everything else remains in dark mode.

### Use Case

Right now, it's easy to miss notification popups. It would be nice to have an option to make them stand out more.

### Compositor(s)

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

### Proposed Solution

A toggle in the options probably.

### Alternatives/Existing Solutions

_No response_

Start by locating the notification popup and options entry points in DankMaterialShell, then check how themes are applied across the listed compositors. Clarify which compositor coverage is required before implementation. Done means an option can switch notifications to a light theme while the rest of the shell remains dark, with the supported compositor scope documented.

Written by the indexing model from the issue text.