[TYPO3-english] Typo3 bootstrap, how to add make available extra layouts for a certain site only?

[P V]

New to Typo3 have managed to install 6.2 without any flaws.
Also multisite cms , have used the bootstrap starter package so far so good.


Now my problem is I would like to create a THEME / template for each of the different sites.
+ Also add some unique layouts to the admin backend.

I can only choose certain layouts that are predefined. How to manually add more layouts?
The BACKEND Layout is also the layout that is displayed in the frontend.

How would i for example create 3x 33% rows then another row with 3 rows and then another with 1 row? Just custom blocks based on bootstrap.

Also how would I create a theme that is hooked to a certain site? the documentation is not clear on this