Help editing "Twelve Fifteen" Theme

Hello –

I have a couple questions regarding the “Twelve Fifteen” theme. My site is https://jeffseltz.com/.

1) How do I edit the font size of the header of the blog posts?

I went into the “Editor” to the “10.0 Header” section, and tried adjusting the “.site-title – font size”, but that didn’t work out. (Keep in mind I’m not a coder).

I also installed the “Easy Google Fonts” plugin, which adjusts the content part of the blog. However, none of the “Header” sections control the blog header.

Please advise.

2) Is there a way to control the width of the side bar? I would like to make it less wide.

3) Is there a way to control the spacing in between the “content” sections in the side bar? For instance, the space between the “Title/Subtitle” and “Subscribe to My Blog”.

Thanks in advance for your time.

Jeff