Need advice for blog layout

This is my first post, so I hope I'm not breaking any rules.

I need help improving the look of my basic website blog. It is at

Currently it uses a softaculous platform called flatpress. It is fine, but I don't think it allows me to change colours or add my own header jpg.

Should I go for basic html, or get a wordpress theme? If a wp theme, how do I find one simple enough that looks like my site but allows a custom header?