[Enhancement]: added css variables for styling

[theinfinull]

exposed overridable CSS custom properties under
.theme-dark and .theme-light for user-styling
via snippets

includes variables for:

• file-status markers for modified and deleted
• diff view line backgrounds/highlights
• editor gutter markers

Docs updated with variable reference

[Vinzent03]

I'm unsure if adding variables is really necessary/the right solution. I would suggest you to write css rules with a higher specificity, which should be possible with for example !important

[theinfinull]

!important works, but it forces users to replicate internal selectors. like overriding the gutter marker size currently needs ~8 !important rules. All of which may silently break if those class names ever change or if new ones are added

Variables collapse em into one line config, and users can style via docs without knowing internal class names, which is how obsidian itself does it.

Seemed like a cleaner approach - open to feedback