There is only one real resolve to this issue unfortunately, and that is to only use a white or extremely light colored explorer background in Windows 10, there is no way else. As far allowing trial periods on a skin, I don't think that is even possible, and if it were, it would no doubt cost Stardock a lot of money to develop encoding procedures. Then they would have to deal with hackers too.

This is why my Windows 10 releases have a white sub-style with them, and all skin designers need to start doing this if they are developing Windowblinds skins for Windows 10.

If you don't want to do as Neil says and want it to be permanent  for that WB use SKS.

I also use Excel, Word and even  Microsoft email. The color that controls those backgrounds is the "Windows Background" color in SKS.

Change skin colors/Edit colors/show me the skin defined system colors/ Window background. Just change the color of the window background, apply and good to go.Note it may be necessary to close excel and open again to see changes. 

Because I do use those programs I set the colors so they can be used. But do the above and very easy.

SKS. Skin Studio. Comes with Windowblinds. Not hard to use for minor changes but you not knowing what it is may not be easy for you. 

Glad you got it. Good job.