To select a theme:
- Open the menu by clicking either the grey button or your picture at the top right of the page.
- Open the settings dialog by selecting the Settings option in the menu.
- Click Background under Visual in the navigation menu on the left.
- Click the Select a theme button.
- In the dialog that opens, mouse over the theme you'd like to install.
- Using the buttons shown, either click Use to install it or Preview to see what it'll look like.
- When you're done, click the blue Save & Close button at the top right of the main settings dialog.