Update custom.css

[jlaehne]

I really like changes to css done on https://www.aiida.net

The ones that I got to work here are:

• Slightly larger font in the top navigation bar
• Less obvious external link symbols in the top navigation bar
• Wider page only for the landing page (otherwise sub-pages like 'about' look too wide)

[ericpre]

• Less obvious external link symbols in the top navigation bar

Is it possible to remove the external link in the top navigation bar? They all stay in the same domain, so in this case, there are incorrect.

• Reformatting sidebar for news-blog
• Adapt post list in news-blog

When I build locally, I don't see any difference for these?

[jlaehne]

Replaced by #55

[jlaehne]

• Less obvious external link symbols in the top navigation bar

Is it possible to remove the external link in the top navigation bar? They all stay in the same domain, so in this case, there are incorrect.

Have not found them yet, but at least they point to the user guide with different navigation bars

• Reformatting sidebar for news-blog
• Adapt post list in news-blog

When I build locally, I don't see any difference for these?

Did not get these changes to work yet, but consolidated the css changes in #55.