Add TabLine* highlights, improve Nvim-Tree visibility, follow background setting, and use main theme colours for Lualine

[Slotos]

Granted, this is a mess of four incremental changes, please tell me if you'd prefer to see them as four separate PRs.

These changes implement, each in a separate commit:

• TabLine, TabLineSel, and TabLineFill highlight groups - something important for anyone who's using tab groups
• Use of background setting, as opposed to forcing it - now colours change as you change your background
• Fix for nvim-tree.lua folders color matching background color in light palette - a color that matches the position of the dark palette was selected instead
• lualine dynamic color switching, using central definition of theme colors - any future updates to this theme will propagate to lualine (lightline and airline would take too much time for me, my apologies)

Here's how it looks now with lualine and a heavily modified luatab:

Dark background

Light background, as a reaction to set background=light

Mish style, with theme reload

[FrenzyExists]

Ok, this is a HUGE update. This color propagation that you made which could reload when theme is changed is what I indeed wanted, but didn't exactly knew how to achieve it. Really appreciate the contribution!