So then why is it like this.
Default Windows 10 Theme:
With Curtain Applied:
This is because Google chrome (and MS Edge Chromium) use a custom titlebar by default. This titlebar has a colour when inactive unlike the rest of the OS. Their tabs are therefore made to be this colour when inactive.
But in order to make them take on Curtains styles the browser is told to use the OS titlebar. This uses a white inactive titlebar so they use white for the inactive tabs assuming that this will blend in.
You can tell Curtains not to put Chrome in this mode BUT you will no longer have themed titlebars in Chrome/MS Edge.
But there is a workaround which we will be implementing in a future Curtains build.
In the registry go to HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\DWM
Create a value AccentColorInactive as a DWORD and set this to in your case probably something like 404040 which is a dark grey
This should instantly take effect. To revert back to default behaviour just delete this value.