Change the theme look and feel to match the ScyllaDB website

[annastuchlik]

Modify the theme to better match the webstie: https://www.scylladb.com/

This is a task for a ScyllaDB designer, so it belongs to the Design_tasks project.

Refs: scylladb/scylladb-docs-homepage#11

@timkoopmans FYI

[dgarcia360]

Some other tasks that the designer might want to take into account:

• Make issue and edit button more visible #621
• Text in version selector should be align to menu #257
• Dark Theme! #251
• Add copyright #358
• add a generic "Tweet about it" button to the theme #859
• Main menu is missing bullets and uses extra spacing #934
• Menu items are hard to expand #945
• the baseline of text in monospace is lower than that of text in regular font scylladb#18905
• redesign the documentation landing page #1171
• mobile: topic boxes do not wrap to next line on landing page #1182
• update scylladb icons #1191