Theme development

Hi. I'm new to Drupal. I just have installed Drupal 10. I'm going to create a website from the theme Olivero(v10.0.9) ; at first I installed non-core themes and used them, but encountered a problem that I could not change themes at all at a certain point, so I re-installed Drupal and decided to use just core theme.

Anyway, now I want to know how to design my website. My concern is whether I should customize css files under /core/themes/olivero or use Starterkit Theme Generator.

To make your Drupal 9/10 forum better looking, we can optimize an output of the forum table when the mobile detect is enabled:

In our sample we will take Bootstrap for Drupal Subtheme - web/themes/contrib/bfd_subtheme/templates/content/forum/forum-list.html.twig

preliminary part -

I'm having some trouble with recompiling themes and the changes not getting picked up by my system.

I ended up manually deleting the noted directory and refreshing the caches and it seems to work.

So, the question is 

If theme recompile and a site refresh all caches does NOT work unless I also delete the noted directory, do I have a misconfiguration somewhere?

Who is creating and compacting these files?

I've been struggling with this for a while and need to break down and ask for help.

The goal is to simply change the Navbar background color in the dark theme of Varbase.

Seems simple enough.  I tried using the gin-custom override in the site/default/files directory but that didn't work.

Now I am trying to create a subtheme of the vartheme_bs4 theme.

How to make group inside group in Theme Setting page? Or wrap some elements inside group. See this image:

https://pasteboard.co/MnAuoD3XPAHz.png

Please help me with this question about overriding libraries:

I have a mastertheme, a subtheme and a subsubtheme
subtheme is a subtheme of mastertheme and subsubtheme is a subtheme of subtheme

in mastertheme.libraries.yml, mastertheme defines the masterlibrary as:

masterlibrary:
  css:
    theme:
      css/theme/file.css
    
in subtheme.info, subtheme overrides this library as follows: