How to Change CSS Colors from Custom Plugin Settings Page

I have a custom WordPress plugin with a variety of settings that I use across multiple websites. I also use a few of the same 3rd party plugins across all of those pages. I would like to be able to stylize them the same way, but with different colors. While I could add CSS to every site, it is difficult for some of our non-techy admins to go through the CSS and change the hex codes on the correct classes. I am trying to figure out the best way to do this. Here is what I’ve looked into so far:

How to create a field in customize and show that in header.php?

I am new to WordPress coding and learning this from stackexchange,
I am trying to create one WordPress website from scratch, in header I want to show contact information for example phone number,
I want to create a field in customize and want to display the value in header.php.
Is there any guide or tutorial for that? Any help will be appreciated. Thanks a lot!!
*don’t want to use plugin