How to create a custom theme for Drupal 9 and Drupal 10 

You can attach specific block to the specific theme in Admin -> Structure -> Block layout You can also attach css file to block, you need to define a library and add to custom block render array.

@@visatit Would you consider making a video tutorial for it with some simple examples? Would be much appreciated. the video would be a more in depth tutorial on how to customize your drupal 10 site using css/html and creating blocks

Thanks for the comment, this video covers topics like - Creating a custom theme - Define the regions - Overriding the twig file - Adding assets (CSS, JS) to a Drupal theme via *.libraries.yml