Rework gtk 3/4 theme

### Feature Description

Since dms is going to adapt to material 3 expressive, it would be cool if all the templates matched that design language. So far, I’ve found these matugen templates for GTK 3/4 in the [MainstreamOS dotfiles](https://github.com/MainstreamOS/dots-hyprland/tree/mainstream/dots/.config/matugen/templates) , and they really match the material 3 expressive style.

Screenshot of nautilus with the new theme

### Use Case

This would make dms and gtk apps more consistent with the new material 3 expressive design language.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution
First, review the templates from the MainstreamOS dotfiles and fix any minor issues, such as using darker variants for better consistency with the other templates and fonts weight. Then replace the current matugen templates with the new one.

### Alternatives/Existing Solutions

_No response_

Start by reviewing the GTK 3/4 matugen templates in the linked MainstreamOS dotfiles and comparing them with the current templates. Check the proposed darker variants and font weights, then replace the current templates; done means GTK apps match the Material 3 Expressive styling across all compositor setups.

Written by the indexing model from the issue text.