Change popup style

[predragnikolic]

closes #1566

This PR makes all the font-size of header elements the same as the other text(1rem).
But header elements are bold, which makes them standout just enough from the rest of the text.

Header font size before:

Header font size after:

---

I also reduced the hr margin to be consistent with the popup margin spacing (0.5)

Before:

After

[rwols]

Looks good to me :)

[predragnikolic]

There was a comment from Rafal :)

I would think that using the same size for all of them could make the content harder to read in case it's structured with multiple different heading levels. - #1566 (comment)

Having different sizes for different headers makes sense for a web page,
but I don't think that having different font-sizes for header would improve readability, because of the limited space in the popup.

[rchl]

I'm fine with trying it but would it make sense to at least make them a little bit bigger than normal text? Maybe 1.1rem for all?

(I can't currently find any content I could test this with so maybe it's not that important either way ;))

[predragnikolic]

There is really a small difference between 1.1rem and 1rem.

I initially chose 1rem because a little more text is visible :)