I'm using Curtains on Windows 10 Preview build 21313.rs_prerelease.210209-1440, which is in beta anyway. I don't expect things to always work perfectly. The last month of Windows builds, Curtains shows weird blue bubbles, extra lines, and the top bar continues beyond the window boundary as you can see in the screenshot.
Please make sure to file a bug using the Microsoft feedback tool in your insider build of Windows.
An investigation here suggests there is a bug in how that Window insider build is rendering titlebars after an accent colour change. If you have the Windows 10 theme applied you can reproduce a similar issue.
To repro without Curtains applied :
1) Apply Windows 10 style from within Curtains
2) Ensure you have an explorer window open next to the curtains UI
3) Click the colours button in Curtains
4) Tick or untick the option to get the accent colour from the wallpaper. You may have to do this a few times or just one may be sufficient (it is timing sensitive and sometimes it fixes itself)
Notice how the shadows on the windows are now only in the corners.
Basically the OS is painting the images using the physical image sizes instead of the window size. As many curtains styles have large images they extend past the edge of the window. As the default OS style has a tiny set of images these paint tiny.
I imagine you could repro the same without Curtains installed by toggling the very same option in the personalisation settings.
Unrelated to this issue, I would recommend you update to Curtains 1.17 beta as that would fix the issue with the weather info on the taskbar having a black background vs the theme one.