Theme development

I'm new to theme development
I don't like the default form design. How can we customize the style of node/add form for the user?

Hi everyone, 

I was wondering how can I change font and color of my theme for all the website in once? 

Should I change it for each component or is there a way that allows me to change it for the whole website? 

NB: Drupal 9.3.13

thank you 

For a new site I wanted to start clean and subtheme the new Claro. Now how do I make it responsive. From the Claro wiki page I read this:

responsive: give emphasis to what matters; use hierarchy to explain the relation between elements; use predictable patterns; elements adapt and react to inputs.

I have been chasing some issues that arose when an update went bad.

I have the site up and operational now on a generic theme, but for some reason the admin theme is not working.  I get the following when attempting anything that uses the admin theme.  Any and all help greatly appreciated.

The website encountered an unexpected error. Please try again later.

bartik design is still as it was in 2008, it's really unusable at this point and cannot be used without creating a child theme and overriding the styling. 

How long do you plan to keep this going?

We have a total of 328 custom blocks in our Drupal 8 site. I'm not sure at which block count (330, 340, or 350), but at a certain point when a new block is added I am unable to move/drag it. I'm having to constantly delete or prematurely expire custom blocks because I reach that threshold. I cannot find any documentation that states there is a limit to how many custom blocks I can have on a Drupal site. Has anyone else experienced this? If so, what is the fix?