Add possibility to modify theme with Obsidian Style Settings plugin

[harmtemolder]

As suggested by @t-molder in #37

See https://github.com/mgmeyers/obsidian-style-settings for more

[harmtemolder]

• [x] Allow for manually overriding colors?
• [ ] Maybe a simple toggle to switch to high contrast dark mode?
• [x] At the very least a toggle to disable highlighting the active line (see #34)

[harmtemolder]

Split open todo into #46