If you are a developer but not really a designer there are a lot of people on ThemeForest making PSD’s looking for people to help convert them into WP themes, you could try reaching out to see if anyone wants to become a partner.
It could be helpful to use something like “Material Design” and follow the design guidelines to create the site and keep everything clean and nice - https://material.io/guidelines/
For inspiration, looking at top sellers is a good idea but also sites like Dribbble provide some good inspiration. You can also search for articles about “web design trends” such as this one - https://envato.com/blog/6-web-design-style-predictions-2017/ - which may provide some more ideas.
Design is one of the hardest parts of making a theme that will not only get accepted but also sell. The reviewers have a keen eye and when they reject your item in some cases even if they had accepted it, it may have not sold. So all you can do is keep working on it and your skills will improve over time Our first theme was rejected like 6 times!
Hopefully this helps a bit, good luck with your portfolio!