There are only 3 core themes:

- vanilla - this is the basis for all other core themes
- bootswatch - this is an extension of vanilla
- fruity - - this is an extension of vanilla

It may be easier to think of an extended theme as a "child" theme. The "extends" value simply indicates a "parent" theme. Any files not found in the extended theme will be pulled from the parent.

If you extend any of those core themes, any files not found in your new theme directory will be loaded the parent theme(s)

bootswatch also has many Bootswatch theme options but it's best to think of these as theme variations - mostly to do with colours.

Regarding the naming, when extending a theme, you have the option to give custom names instead of the default naming structure.

More info here - www.limesurvey.org/manual/New_Template_System_in_LS3.x

You can see what theme is active for a given survey in the survey General Settings.