Hello,
Since you are trying to create a simple theme. Than, you just need to create a simple graphics with two squares on-top of each other. The top is for active state and the bottom for inactive state. Make it transparent to any degree that look good to you. Import it into Curtains. Something like the screenshot below.

Basj,
Stardock Community Assistant.

Hello, thanks for the information. 

I also just want to make the titlebar and the window borders a bit transparent in the default theme named "Windows 10 by Microsoft" in Curtains (I'm using a Vista inspired Windows Theme).

If I use two squares on top of each other with different transparencies (see attached), it seems to work to some extent. However, the top corners of the titlebar then lose their roundness. Also, the frame doesn't become transparent?

How can I avoid this, i.e. how can I add just some transparency to Titlebar and window frame, but keep everyting else like colors and round corners? 

<- Here's the transparent squares image

Hello,

If you interested to change the titlebar from someone design. What you can do is to open the graphics of that part and edit it in a graphics program such as Adobe Photoshop or something similar. Give it less transparent or more transparent. Save it somewhere and import it back into Curtains.

Thank you,

Basj,
Stardock Community Assistant

No, Curtains doesn't have that feature.  It merely uses whatever transparency is built into the image(s).

You need to build the rounded corners into the images.

WindowBlinds does have a user slider control which allows you to adjust the Window frame transparency (caption and borders together) directly within it, and on an individual skin basis, without editing the images.

If you want blur on the titlebars you will need to use WindowBlinds - Curtains doesn't support titlebar blur.

Otherwise the quickest comparison of the two in terms of creating a skin is:  WindowBlinds is much more thorough with more skinning control and capability, and more complicated.  Curtains is much simpler and more limited.