Setting a Theme within the Chrome Extension
While you're working on the Chrome Extension, you can set a custom theme to a guide either before or after creating a guide, as shown below:
Before creating a guide:
After creating a guide:
Do not forget to hit the Publish Changes button on your panel to make your changes live.
Setting a Theme within the Panel
You can use Advanced Settings of your Guide/Hotspot to change the Theme, as shown below.
Why is the Custom Theme not applying to my Guide/Hotspot thoroughly?
If you change the appearance of a guide or hotspot manually while working on the Chrome extension, they are considered retouches.
And, retouches within individual steps can override the functioning of the Theme that is designated to a guide, and they will not stick to the guide completely.
If you want to thoroughly apply your changes to the guide, you need to change the guide's Theme, not the step-by-step design.