It looks pretty good. I think I might have a few suggestions that you can take or leave!
1. The contrast on the background image is a bit much, I think. I'd reduce the contrast so that the background doesn't stand out so much.
2. I think you need an accent colour. If it was me, I think I might go for a nice deep blue or even green. And then just splash it around sparingly. Don't overdo it, but just introduce a little bit of colour here and there.
3. You might want to also introduce some responsiveness to the theme. It's in a pretty easy state to transition from, really. You could just pop the sidebar beneath the content once the window is small enough, and then you're pretty much mobile friendly as well!