I downloaded and applied your "Sonix" Skin, it has the same "Minimize Button" issue when you try to minimize the window, click twice to minimize window. This issue is not on all application, notepad, Microsoft Edge and explorer.exe they worked fine, just a single click, same behavior with all the Skins I've tried.
Today, I have finally tried every Local Skin that are installed by default when you install the Curtains application, and sadly, every Skin has the same "Minimize Button" issue.
If you haven't experienced this issue, please let me know what kind of computer are you using, or do you know if certain versions of Windows 10 aren't fully compatible with some Skins? or if there is some patch for it, or something else related.