If you do not have a custom theme, follow these steps to create it:
1. Go to Customization > Manage Templates.
2. Notice the sidebar on the left and remember which theme is the current one (Theme1, Theme2, or Theme3).
3. Click on the green button with with + on it to add a new theme.
4. Provide a name for your custom theme (any name would be just fine) and select a theme to be taken as base theme - select the theme which you remembered on the step 2.
5. Click "Save & Close".
Now you have a custom theme. To apply it:
1. Go to Customization > Styling and Front-end Settings.
2. Select your new custom theme from the list.
3. Click "Save".