Redesign (update) site and CSS change

My site is almost as old as Weblfow :slight_smile: and requires some design and style updates
I have tons of pages and styles created
I was wondering if somebody can advise on the best way to update the site?
For example, what should I do if I want to change the font family throughout and update pages to a more current design style?
Thank you!

sorry forgot to add the site:

Here is my site Read-Only: LINK
(how to share your site Read-Only link)