Add Theme Support For Materialgram App

### Feature Description

Add Support To Use DMS's Accent Color, Similar To Other Apps.

### Use Case

More Native Looking To The Desktop

### Compositor(s)

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

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

No file, test, or entry point is identified in the issue. Start by locating the Materialgram app's theme handling and how DMS provides its accent color; clarify the expected behavior and verify that the app presents a more native desktop appearance across the relevant compositor setups.

Written by the indexing model from the issue text.