On the Mac:

1. Go to your Applications folder
2. Find ada.app (on your Mac it might not have the .app at the end)
3. Right-click (or Control-click) on ada.app to get the popup menu
4. Select Show Package Contents:
   
   
5. Navigate into Contents > Resources > assets > themes
6. Inside the themes folder you will see a list of folders which are the themes that come pre-installed with Ada.
7. To make your own custom theme, first make a copy of the ada theme folder and make it's name the name of your new theme. Open the folder you just made and you will see a folder of images and a style sheet. The style sheet is commented so you should be able to figure out how to customize it. To see your changes you will have to restart Ada. That's it!

On Windows:

1. Go to your Program Files folder. This is usually at C:\Program Files.
2. Inside the Program Files folder find the ada folder.
3. Navigate into ada > assets > themes.
4. Inside the themes folder you will see a list of folders which are the themes that come pre-installed with Ada.
5. To make your own custom theme, first make a copy of the ada theme folder and make it's name the name of your new theme. Open the folder you just made and you will see a folder of images and a style sheet. The style sheet is commented so you should be able to figure out how to customize it. To see your changes you will have to restart Ada. That's it!