Redsign HTML tags and buttons

Hello,
I’m working on the design of my website,
And I want to chage the style of the html tags (H1 H2 etc) and the bottons style.
I see in the avia builder the options of design but it so limited.
Is there any other way then just adding new css styles to style.css of the child theme?

It takes me so much time only to find the css and change
(I’m a designer not a developer)

And if so, how can I put all the design elemets i one page so i will be able to see the changes?
I mean, I want to redesign all of the components of the website and to see the effect in one page.

Thank you very much,
Eli