These changes can only be made in CSS, so you need some basics.
you can use your browser’s “inspector” tool (F12) which will help you target your css rules and make your changes live.
You will then have to transfer these rules to the style.css file of your theme.
Be careful, download your theme BEFORE so that you have a backup.
Your need can be very sensitive depending on your level of html/css knowledge, and can even cause some problems.
It will be very difficult to help you.
In the same way that an electrician could not help me by telling me what to do (since I don’t know much about electricity).
sorry for my English, it’s not my native language, I think that if these changes are important to your site and your brand, you should list them on a google doc, and find a professional who can help.