Updates logo scss to regard default values and updates favicon upload check

[weeman1337]

Logo
With the change from here: e8938df the scss variables from theming were overwritten by the value in the variables.scss.

This fix adds support for default values to the logo and background scss.

Favicon
SVG favicons are not working if they're SVG and imagemagick isn't present. There is already a notice on the theming page, but the user is still able to upload a SVG favicon. I changed this so the upload returns "unsupported image type" for SVG favicons without imagemagick.

Closes #10957

[weeman1337]

For me the failed tests look unrelated. But someone should double check.

[skjnldsv]

I restarted the tests. It looks unrelated yes :)

[juliusknorr]

@rullzer Any chance we can still include 84fa208 into the 14 release? Because otherwise the theming of logos is broken 🙈 Other option would be to revert e8938df and include both fixes in 14.0.1

[rullzer]

@juliushaertl yes sure if it is a regression lets do it. Changes are minor it seems.

[juliusknorr]

All right, created a backport PR for the first commit, since that is the important one to fix the regression: #11048

[rullzer]

Rebased

[nickvergessen]

@weeman1337 do you know, should this go into 14 too?

[weeman1337]

@nickvergessen I think #11055 should go to.
@juliushaertl closed it. Maybe it's solved by another PR?

[MorrisJobke]

@nickvergessen I think #11055 should go to.
@juliushaertl closed it. Maybe it's solved by another PR?

@juliushaertl Any comments why you closed it?

[MorrisJobke]

@juliushaertl 🏓 ;)

[juliusknorr]

I closed it since @rullzer already opened a correct backport request for that PR in #11052