Minor style fixes and adaptions

[awinterstein]

The following fixes were done to the styling of the theme:

• Added missing background definition for light theme to mobile menu (the background was transparent before).
• Wrap categories and tags lists if there are too many items for one line.
• Align page content in the center so that it does not look off when the table of content is disabled.
• Always use current year for the copyright notice in the footer

The following adaptions were done to the styling of the theme:

• Distance to the page borders for content blocks which is especially helpful for mobile layouts, where the content might touch the screen borders otherwise.
• Moved content on index page more up so that it does not look off on small mobile screens or with multi-line content text on the page.
• Restrict width of the content div on the index page so that the line wrap happens for larger content texts before the text runs over the whole screen.

I tried to test that the changes do not break your pages by building it with your website repository (https://github.com/tchartron/tchartron-zola/). However, the build fails due to issues unrelated to my changes. Please let me know, if I can test that somehow.

[tchartron]

Sorry for the delay but it's holiday time 🎉
I checked the changes on my phone and it looks pretty safe to merge, thanks a lot for the contributions !
I will try to rebuild my personal website but I haven't maintained it for a while now ...