There is the option of choosing a theme by category. It is not advisable to work with it since there is a warning tip because it may affect performance. Category themes affect other course themes:
Under Site administration, click on Appearance | Themes | Theme selector.
Click on Allow category themes if you consider that it is necessary; otherwise, it is not advisable. It is shown in the following screenshot:
Then scroll down the page and click on Save changes.
Tip
When selecting another theme in order to be detected by different mobile devices, smartphones, desktops, and so on, we have to click on Enable device detection.
Under Site administration, click on Appearance | Themes | Theme settings. Click on Enable device detection, as shown in the following screenshot:
Click on Save changes.