Change content heading background to bg-body-tertiary

[AntonKhorev]

This is related to #4645 where I wanted to make the background of one element to match exactly the heading background in order to make other element below it invisible. Then I discovered that the heading background color is not a standard Bootstrap color and therefore I'd have to do more custom css if I wanted to reproduce this color.

Why not pick some standard color for the heading background? One advantage for it can be seen here: #2332 (comment). If dark mode is enabled, the current color doesn't match the theme and you'd have to come up with another custom background color for heading sections. Standard colors have dark mode versions predefined.

Which standard color is it better to use? The closest one seems to be bg-body-secondary but I'm using here bg-body-tertiary which is noticeably lighter. That's because there's another problem with the current color which becomes worse with bg-body-secondary.

Sometimes we display tabs over the heading background. Those tabs have a border when hovered. However the color of that border is very close to the background color, making it difficult to see it:

With bg-body-secondary you won't be able to see the border at all. But with a lighter background that I set in this PR it's easier to see the border:

[tomhughes]

I understand the problem but I don't think this solution is going to work...

The tertiary colour is so pale as to be next to invisible so I don't think it's going to work and even with the paler colour the tab hover is still hard to see.

I think we should probably go with secondary and then find some better way to do the tab highlight, which I hadn't even realised existed until now!