Formatting the sidebar and page template

My blog (https://www.thegoldenhat.com) uses a really simple template that is perfect for being content-heavy. I’m a novice at php and html and was curious how I can go about changing the color of my sidebar, links in my sidebar, and font in the sidebar to try and add a bit of style to my template.

Does anyone know where I can do that in the php code and where I can get a list of the color scales, fonts, etc that I can test out?

Thanks everyone!