If you change the margin-top it should definitely decrease the space. That’s where the space is coming from. For example, when looking at your site through my inspector, I added in a sample widget area with the after-header-widget CSS class to see how it would appear. This is what it looked like at first – https://i.postimg.cc/t4hmhjWz/Screen-Shot-2019-08-23-at-4-13-07-PM.png
Once I decreased the margin-top CSS I mentioned earlier, the space reduced and looked like this – https://i.postimg.cc/xd5x43sJ/Screen-Shot-2019-08-23-at-4-13-36-PM.png
Once you make that change or any change to your CSS file make sure to clear your Browser Cache so you can see the results appear faster (This is better than waiting on the browser to clear it on its own). I usually just click the Reload button on my browser and that usually does it.
If you’re using a Caching plugin then you may need to clear the cache from the plugin itself. Other than that, if I’m still not seeing the changes then I would go into the browser menu and clear the cache from there directly.
Let me know if that works.