You can update the styling on all of your website headlines in the main.css file under Theme > Edit HTML / CSS (button in lower left). I would recommend copying the existing main.css file (paste it into a notepad doc and save it) before making any changes. This way if you change something and aren't sure how to get back to where you were you have somethin to fall back on.
Once you have saved a copy of your existing css file, simply scroll down and look for the H1 section in the editor, and adjust the font, font size, padding, etc. to your liking. Same for H2, H3, H4, H5, etc. There may not be any styling associated with some of these subheaders. If there isn't, you can just add them below the existing H1 / H2 styling in that main css file.
Weebly's editor shows you a preview of the page you're looking at in real time while you make the updates, so navigate to a page with all of ther elevant tags you'll be updating before going to Theme > edit HTML / CSS so you can view the changes as you make them to confirm it looks how you want it to.
Hope this helps!
