Regenerate/Update GTK theme (themix-gui 1.15.1)

The Materia version that themix-gui-1.15.1 uses is still from May 12, 2021

Some notible changes is that I had to switch from inkscape to resvg
since exporting using inkscape has been borked since 2021. Hopefully
things still render properly.

Issue on themix-gui's repository regarding inkscape and materia.
themix-project/themix-gui#387

How I got materia to export with resvg.
nana-4/materia-theme#591

themes/Gruvbox-Material-Dark-HIDPI/INSTALL_GDM_THEME.md

@@ -0,0 +1,47 @@
+# How to change the GDM theme
+
+To change the GDM (lock/login screen) theme, you need to replace the system's _default_ GNOME Shell theme.
+
+## :warning: Cautions :warning:
+
+- If the replacement fails, your desktop environment will not work properly. So please **be careful** if doing this.
+- When applying this, other third-party GNOME Shell themes would look broken until you restore to the original theme.
+- If GNOME Shell has been updated and restored to the original theme, you will need to install this again.
+  > Tip: On Arch Linux, you can automatically reinstall the GDM theme by creating a [hook](https://wiki.archlinux.org/index.php/Pacman#Hooks) file. For example, if you use Materia-compact as your GDM theme, put [this file](https://gist.github.com/looselyrigorous/a1aa931ce48627cc39daccc9253b30c7) in the `/usr/share/libalpm/hooks/` directory.
+
+## Requirements
+
+- `glib-compile-resources` — The package name depends on the distro.
+  - `glib2` (Arch Linux)
+  - `glib2-devel` (Fedora, openSUSE, etc.)
+  - `libglib2.0-dev-bin` (Debian, Ubuntu, etc.)
+- `libxml2-utils` — Ubuntu users only.
+
+## Installation
+
+1. **Back up** the original theme file first. If you are already using a custom GDM theme, skip this:
+
+  ```sh
+  sudo cp -av /usr/share/gnome-shell/gnome-shell-theme.gresource{,~}
+  ```
+
+2. Select a **GTK** theme to decide which variant to install.
+3. Replace the default theme file:
+
+  ```sh
+  THEME_NAME="$(gsettings get org.gnome.desktop.interface gtk-theme | sed "s/'//g")"
+  THEME_SRC_DIR="/usr/share/themes/$THEME_NAME/gnome-shell"
+  sudo glib-compile-resources --target="/usr/share/gnome-shell/gnome-shell-theme.gresource" --sourcedir="$THEME_SRC_DIR" "$THEME_SRC_DIR/gnome-shell-theme.gresource.xml"
+  ```
+
+4. Reload the theme. If you are running GNOME Shell, press <kbd>Alt</kbd> + <kbd>F2</kbd> then type `rt`.
+
+## Uninstallation
+
+1. Restore to the original theme file from the backup:
+
+  ```sh
+  sudo mv -v /usr/share/gnome-shell/gnome-shell-theme.gresource{~,}
+  ```
+
+2. Reload the theme. If you are running GNOME Shell, press <kbd>Alt</kbd> + <kbd>F2</kbd> then type `rt`.