Easy, existing GTK themeing in Firefox/Thunderbird + requested GTK fix

### Feature Description

There's an option in `about:config` that works to instantly theme firefox to better match the GTK theme, with no additional work. However, the existing matugen GTK template seems to be missing some relevant fields.

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NOTE: I'm using the most recent versions of Firefox and Thunderbird, running niri on Arch Linux, with `adw-gtk3` theme and the most recent git version of DMS.

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Anyway, by setting `widget.gtk.libadwaita-colors.enabled` to false, Firefox and Thunderbird both take up my GTK theme in a very pleasing way, at least for the dark theme. Here's the before:

And the after:

Anyway, that's just my PSA about this config flag. The request is for the matugen GTK template to be modified so that firefox's accent color for text highlight and button focus follows the system theme rather than the original blue `adw-gtk3` color.

### Use Case

You can see in the after image (though this is true in both cases) that the original blue adwaita accent color is being used by firefox for text selection and element focus, as well as check boxes and radio buttons. This seems to be specific to `adw-gtk3` themed by matugen. Other GTK themes are correctly followed, both those with fixed colorschemes and also those themed using `wpgtk`, which I was using before moving to DMS.

I do recall also that this may be an issue for the two existing Firefox theming options in the DMS docs.

### Compositor(s)

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

### Proposed Solution

I think this should just involve adding the appropriate color definition to the matugen GTK template, but I'm not sure which one specifically. I've played around with this a bit with zero success. If anyone out there knows, please jump in.

### Alternatives/Existing Solutions

I can't recall if Pywalfox with userChrome and userContent had this issue, but it's possible that some combination of css modifications would fix this. I've also tried Zen browser with the DMS `userChrome.css`, and it does have the same issue, though it's more limited.

Locate the existing matugen GTK template and inspect its generated colors with Firefox and Thunderbird using widget.gtk.libadwaita-colors.enabled set to false. Compare the text-selection, focus, checkbox, and radio-button colors against the system theme; done means those accents follow the generated GTK theme instead of the default blue.

Written by the indexing model from the issue text.