Document 3rd party theme installation in "Advanced Usage"

docs/advanced-usage.md

@@ -12,3 +12,57 @@ These advanced settings can be accessed by the following procedure:
 1. Select the "**Preferences: Open Settings (UI)**" command from the menu.
 
 This will open a "**Preferences**" view in the IDE. Once you are finished adjusting settings, it can be closed by clicking the **X** icon on the "**Preferences**" tab.
+
+## 3rd party themes
+
+Arduino IDE is built on the [**Eclipse Theia** IDE framework](https://theia-ide.org/), which allows it to use [**VS Code**](https://code.visualstudio.com/) themes.
+
+---
+
+❗ Arduino does not maintain or provide support for individual 3rd party themes. If you experience problems with a theme, report it to the theme's maintainer.
+
+---
+
+### Obtaining themes
+
+A large selection of free VS Code themes are available from the **Visual Studio Marketplace** website:
+
+1. Find a theme you want to install: <br />
+   https://marketplace.visualstudio.com/search?target=VSCode&category=Themes
+1. Click on the theme to open its extension page.
+1. Click the "**Download Extension**" link on the right side of the extension page.
+1. Wait for the download to finish.
+
+### Installation
+
+1. If Arduino IDE is running, select **File > Quit** from the Arduino IDE menus to exit all windows.
+1. Create a folder named `plugins` under Arduino IDE's configuration folder:
+   - **Windows:**
+     ```text
+     C:\Users\<username>\.arduinoIDE\
+     ```
+     (where `<username>` is your Windows username)
+   - **Linux:**
+     ```text
+     ~/.arduinoIDE/
+     ```
+     ❗ The `.arduinoIDE` folder is hidden by default in the file manager and terminal.
+   - **macOS:**
+     ```text
+     ~/.arduinoIDE/
+     ```
+     ❗ The `.arduinoIDE` folder is hidden by default. You can make it visible by pressing the <kbd>**Command**</kbd>+<kbd>**Shift**</kbd>+<kbd>**.**</kbd> keyboard shortcut.
+1. Copy the downloaded theme file to the `plugins` folder you created.
+1. Start Arduino IDE.
+1. Select **File > Preferences** from the Arduino IDE menus. <br />
+   The "**Preferences**" dialog will open.
+1. Select the name of the new theme from the "**Theme**" menu in the "**Preferences**" dialog.
+1. Click the "**OK**" button.
+
+### Uninstall
+
+If you later decide you would like to remove a 3rd party theme you installed, it can be done by following these instructions:
+
+1. If Arduino IDE is running, select **File > Quit** from the Arduino IDE menus to exit all windows.
+1. Delete the theme's `.vsix` file from [the location you installed it to](#installation). <br />
+   ⚠ Please be careful when deleting things from your computer. When in doubt, back up!