Rollup merge of #74361 - GuillaumeGomez:theme-logo, r=Manishearth

Improve doc theme logo display

Fixes #74350.

The first commit cleans up the whitespaces and converts them to tabs. We should definitely write a tidy check for this (will do it in another PR).

Screenshots:

![Screenshot from 2020-07-15 14-08-25](https://user-images.githubusercontent.com/3050060/87543748-8581c800-c6a5-11ea-8417-cbf98ebbfd10.png)
![Screenshot from 2020-07-15 14-11-59](https://user-images.githubusercontent.com/3050060/87543747-84e93180-c6a5-11ea-8cea-976b1470e809.png)
![Screenshot from 2020-07-15 14-12-12](https://user-images.githubusercontent.com/3050060/87543745-84509b00-c6a5-11ea-8324-c3c46ab2d9ef.png)

r? @lzutao
cc @Cldfire

diff --cc src/librustdoc/html/static/themes/ayu.css
Simple merge

diff --cc src/librustdoc/html/static/themes/dark.css
Simple merge

diff --cc src/librustdoc/html/static/themes/light.css
Simple merge