Good artists steal. I know most things I have worked on we've taken different visual concepts from various sites and then incorporate them. The key is to steal IDEAS not actual content, and ideally merging different good ideas to create an original feel. After that you just utilise the feedback loop to polish what you have into something that the majority of your users like.
After you've done the above a handful of times you start to get a gut feeling for things which you should listen to as it will often be more right than more wrong.
I suspect a lot of people have a similar-ish work pattern. I'm certain there are a few visionaries that legitimately create new ideas, but the majority of us are just recycling old ideas in a new way.
I've been doing this for almost 15 years now. I definitely create new things, but I didn't start that way. And really, you should use conventional things for most problems that you come across. It takes a pretty unconventional problem to justify an unconventional solution.
This is the same advice I was going to give and is how I've been working on my design skills.
One addition: you should look through Dribbble [0] or similar sites for inspiration, and you can use pre-existing resources for Sketch or Photoshop [1] to build mock-ups. I find it better to improve on design by iterating a bunch on mock-ups before working in HTML/CSS/iOS (since I'm much more familiar with going from design mockup -> front-end).
After you've done the above a handful of times you start to get a gut feeling for things which you should listen to as it will often be more right than more wrong.
TL;DR: Light theft and practice.