In a couple of weeks I will have a forum for answering this kind of question. The forum is for a business, but we give away lots of free samples. In the meantime, most of what you are trying to do you can figure out yourself by playing with Chrome Developer Tools Chrome DevTools Overview - Google Chrome.
The basic idea is to open up the DevTools window on the page you are trying to change. You can then change your site’s css and html inside the DevTools window until you have it looking the way you want. If you are working with Discourse, all you have to do from there is copy your changes into the Discourse custom CSS/HTML editor.
It might be worth making a ‘howto’ topic on this site for how to customize Discourse with Chrome DevTools.