Overview panel renders as stuck/glitched blur box in Frame layout (v1.6.0) — invalid var() in generated gtk-3.0/gtk.css

## Description

After updating to v1.6.0 ("Marble Tabby"), the Overview quick-settings panel renders as a stuck/glitched blurred box instead of showing its content (clock, weather, calendar, media, settings shortcuts). Reproduces reliably with `Layout: Frame` in Dank Bar settings.

## Root cause found

`~/.config/gtk-3.0/gtk.css` (generated by the DMS matugen GTK template) contains one rule that uses GTK4-only CSS custom-property syntax (`var(--x)`) instead of the GTK `@define-color` reference syntax that every other rule in the same file uses:

```css
/* Dialogs */
window.background.dialog, messagedialog, .dialog {
background-color: var(--popover-bg-color);
color: var(--popover-fg-color);
}
```

This should be:

```css
window.background.dialog, messagedialog, .dialog {
background-color: @popover-bg-color;
color: @popover-fg-color;
}
```

GTK3 does not support CSS custom properties (`var()`/`:root`) at all — that syntax is only valid in `gtk-4.0/gtk.css`, which legitimately defines `:root { --popover-bg-color: ...; }` and references it with `var(--popover-bg-color)` (see lines 2-21 of the generated `gtk-4.0/gtk.css`). It looks like the new "Dialogs" block was added to both the GTK3 and GTK4 templates, but the GTK3 version was copy-pasted from the GTK4 one without converting the syntax.

Because the whole rule fails to parse, GTK emits:

```
(qs:5445): Gtk-WARNING **: 16:05:30.978: Theme parsing error: gtk.css:328:23: 'var' is not a valid color name
(qs:5445): Gtk-WARNING **: 16:05:30.978: Theme parsing error: gtk.css:329:12: 'var' is not a valid color name
```

Whatever native GTK dialog/widget the Overview panel embeds fails to theme itself as a result, and the panel is left showing a stale/garbled blurred capture instead of its content.

## Steps to reproduce

1. Update to DMS v1.6.0.
2. Set Dank Bar layout to `Frame` (Settings → Layout → Frame).
3. Open the Overview quick-settings panel.
4. Panel shows a stuck, blurred, glitched box instead of the clock/weather/calendar/media content.
5. Check logs — the `Gtk-WARNING` lines above appear when the theme is (re)applied.

## Expected behavior

Overview panel renders normally, as it did before v1.6.0 (see attached screenshot of correct rendering).

## Environment

- DMS version: v1.6.0 ("Marble Tabby")
- Compositor: Hyprland
- Distro: Fedora
- Layout: Frame

Start by locating the DMS matugen GTK3 template that generates ~/.config/gtk-3.0/gtk.css, then compare its Dialogs block with the GTK4 template described in the issue. Done means the generated GTK3 stylesheet applies the GTK3 color-reference syntax, the reported Gtk-WARNING lines are gone, and the Overview panel renders its content normally in Frame layout.

Written by the indexing model from the issue text.