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+## How to create a new theme
+
+It's very easy to add a new theme, there are just a few simple steps:
+
+ 1. Add a new `<option>` element to `select#theme` in [`src/web/html/index.html`](https://github.com/gchq/CyberChef/blob/master/src/web/html/index.html) with the name of your theme.
+    
+    ```html
+    <option value="mytheme">My Theme</option>
+    ```
+
+ 2. Copy the contents of [`src/web/stylesheets/themes/_dark.css`](https://github.com/gchq/CyberChef/blob/master/src/web/stylesheets/themes/_dark.css) into a new file with the name of your theme i.e. `src/web/stylesheets/themes/_mytheme.css`.
+ 3. Modify the JSDoc comment appropriately.
+ 4. Change the class name attached to the `:root` selector to be the same as the value you gave the `<option>` tag in step 1.
+    
+    ```css
+    :root.mytheme {
+    ```
+
+ 5. Change the values of the CSS properties to modify the theme to your taste.
+ 6. [Submit a pull request!](https://github.com/gchq/CyberChef/wiki/Contributing)