When I open the webdevelopment tool
I see this (in theme "bootswatch")
As you see the background color is set to "maroon.
Now when you investigate the navbar you see that here the class ".bg-light" is affected
Adding both to the "custom.css"
Or you address the navbar and other elements directly like
Code:
.navbar {
background-color: yellow !important;
}
to get this
I do not see a real issue.
And at the beginning you showed "bootswatch". But now you talk about "vanilla".
Ths is different.
As you will find out the "question-title-container" has no added class "bg-primary". So you have to set the container's background color.
Code:
.question-title-container {
background-color: yellow !important;
}
And fruity might be different as well.
Often you have to specify it, like
.fruity .bg-light {..
So investigate and try.
Joffm