Remove unused hybrid.css, rename code style theme file

[nnja]

There were two css files present containing themes for code styles:

• static/css/hybrid.css
• static/css/atom-one-dark-reasonable.css (actively used)

In this PR I:

• removed the unused hybrid.css
• removed references to hybrid.css
• renamed the css file to the more reasonable code-style-theme.css

[McShelby]

From a CSS point of view hybrid.css is still loaded but its styles may be overwritten by atom-one-dark-reasonable.css. I am not quite sure if the later will redefine all of the previous styles.

Nevertheless, I think it's not a good idea to load two highlight.js styles in one html file. So deleting hybrid.css and all its references may be a good idea (it's also still in use in the `404.html')

The files shouldn't be renamed, because that's their official name in the official hightlight.js repo and therefore would hide their origin.