[FEATURE] Customizing GTK Window Borders with CSS for Enhanced Window Decorations

I made a modification to the GTK window decorations by adding borders and customizing their appearance using CSS. The main changes include defining custom properties for the border width, color, and radius, which are applied to various types of windows. Here's a breakdown of the modifications:

```css
/*********************
* Window Decorations *
*********************/
window {
--wm-border-width: 3px;
--wm-border-color: #6272a4;
--wm-border-radius: 12px;
}
window.maximized {
border: var(--wm-border-width) solid var(--wm-border-color);
border-radius: var(--wm-border-radius);
}
window:not(.maximized) {
border: none;
}
window.csd {
margin: 0px;
border-radius: var(--wm-border-radius);
border: var(--wm-border-width) solid var(--wm-border-color);
}
window.csd.popup,
window.csd.dialog.message {
border-radius: 12px;
border: 4px solid var(--wm-border-color);
}
window.solid-csd {
margin: 0;
padding: 4px;
border-radius: var(--wm-border-radius);
border: 4px solid var(--wm-border-color);
}
window.popup,
window.ssd {
border-radius: 12px;
border: 4px solid var(--wm-border-color);
}
```


Start by locating the GTK theme CSS entry point where window decoration selectors are defined, then compare it with the selectors and custom properties shown in the issue. The work is done when the requested border widths, colors, radii, and window-state behavior are applied consistently and the affected windows match the attached examples.

Written by the indexing model from the issue text.