It's the default Windows 10 theme with a dark midnight blue color. The color isn't normally allowed so I used Winaero Tweaker to force Windows to use it, but using the allowed ones like navy blue didn't fix the problem, and I love this color too much to switch to another. Lol. Downgrading to 1.17 gave me back my white active tabs.
Btw, my Notepad++ looks like this because of the settings I chose (Tab Bar and toolbar hidden, no border edge, etc.), not because of the Windows theme.