How can I improve my design skills?

Color theory is a big topic but there are lots of tools out there to help narrow down your color pallet and make sure the colors go together. Here's a random one I picked from search results, there are tons of these though.

My personal preference on font size is a 16px base and then I use ems to scale up/down from there for paragraphs, headers etc. That's not to say it's the "right" way necessarily but I've gotten good results with that workflow.

It's good to avoid "samey-ness" but you still want the navigation to be familiar enough that it can be used without explanation. Website visitors have come to expect a site to have controls work in a predictable way so some conventions are good to follow on things like menu navigation. Some designers go overboard trying to be so out-of-the-box that they end up with a site that looks really cool and unique but is totally unintuitive to use.

Striking a balance between artistry and usability is a tricky dance. One of the best ways to mitigate the guesswork is knowing your audience. A key question you need to ask at the beginning of any software project is who is this for? That will help you decide what content to prioritize and how deep in the navigation each piece of content should be.

UX-wise, two of my favorite books on the subject are:

• Don't make me think (revisited) - Steve Krug
• Designing With the Mind in Mind - Jeff Johnson