Works only partially on my site

Hi! Thank you very much for the D8 port!

It seems that I miss something - perhaps it is because I do not understand what the second installation step "Place a block attributes" mean...?

After activating the module, I see a field to enter a CSS class, but only on system blocks, not on user defined blocks (which would be much more important for me)!
If I define a CSS class for the powered-by-drupal block or the content block or the page title block, the class is not added to the HTML code. If I add a class to the search block, it seems to work perfect - but only if I use the bootstrap theme as theme. When I use Bartik, I see no class in the HTML code...

I have installed Drupal 8.03.

What am I doing wrong? Any help is appreciated!

Many thanks, Michelle

Hi! In the meantime I have realised that I have to set the block class on the block layout page after assigning it to a region instead of trying to assign a class in the library of user specified blocks... That works!