Document 3rd party theme installation in "Advanced Usage"

Arduino IDE comes with a selection of officially maintained and supported built-in themes:

- Light
- Dark
- High Contrast

Although these three should be sufficient for most users, some users may have other requirements or preferences for the
Arduino IDE UI theming. Fortunately, because it is built on the Eclipse Theia IDE framework, Arduino IDE supports VS
Code theme extensions. This makes a large variety of 3rd party themes available to the user, and even the ability to
create custom themes.

Instructions for installing VS Code theme extensions are here added to the "Advanced Usage" document hosted in the
repository.

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!