Update docs to: Add theme changing support at runtime (#100)

Author: github-actions[bot]

docs/themes.md

@@ -0,0 +1,42 @@
+# Themes in Text Forge
+Text Forge supports customizable themes for UI elements. This page explains how themes work, how to 
+create your own, and how to install or share them.
+
+## Theme Structure
+Each theme is defined as a `.tres` file. This file is a Godot (text-based) resource in INI style.
+You can style UI elements with this file.
+
+## Getting Themes
+Text Forge has some internal themes:
+
+- Dark (default) `dark`
+
+You can find other themes here:
+
+- [Text Forge awsome themes](https://text-forge.github.io/awsome-themes)
+
+## Installing Themes
+To install a theme, open **Settings > Open Data Folder**, go to the `themes/` folder, and paste the
+`.tres` file there.
+
+!!! Note
+
+    You can use installed themes without restarting the editor.
+
+## Changing Editor Theme
+To change the theme, go to **Settings > Preferences > Editor UI > Theme Name**, enter the theme file
+name (without the extension), and close the Preferences window. The editor will load the new theme.
+
+## Creating a Theme
+If you want to create themes, use the Godot editor. See the guide for opening the Text Forge source
+in Godot:
+
+- [Build Text Forge from source](build.md)
+
+Then edit `res://data/themes/dark.tres` in the Godot Theme Editor or create a new theme in that
+folder. When your theme is complete, rename it and share it. You can test it directly; the editor
+copies `res://data/themes/` to `user://themes/` when you run it.
+
+## Themes Location
+The editor loads all themes from the `themes/` folder in the editor data directory. Internal themes
+live at `res://data/themes/`.

mkdocs.yml

@@ -26,6 +26,7 @@ nav:
       - Setup: setup.md
   - Guides:
       - Menus: menus.md
+      - Themes: themes.md
   - Architecture:
       - Data Driven UI: data_driven_ui.md
       - Object Oriented Design: object_oriented_design.md