Themes & Colours: delete or edit a saved theme

Hello everyone,

I'm a little confused about themes, master slides, and theme colors.

In the attached sample slides and images, you can see that a variety of themes are listed under "Design" - "This project." If I switch to the master level, I also find a variety of slide masters there. My goal, however, would be to only use the theme under "Custom." What do I have to do for that?

I'm having a similar problem with the "Design" - "Colors" section. There, under "This project," I see two color blocks that I'd actually like to delete and replace with the custom theme colors "Diakonie." How can I delete or edit the preset themes or colors?

When I right-click, there's only the "Apply" option, but not the "Delete or Edit" option.

I'm happy to hear any tips ;-)