Feature request: Full theming

[RobertKosten]

There's a lot we currently cannot change using the YAML theme files (the built-in themes can "cheat", e.g. via variables, if I'm interpreting the code right), it'd be nice to have full control, especially of foreground colors.

Given an "all black" yaml

---
name: all_black

primary: '#000000'
secondary: '#000000'
background: '#000000'
surface: '#000000'
panel: '#000000'
warning: '#000000'
error: '#000000'
success: '#000000'
accent: '#000000'

text_area:
  gutter: '#000000'
  cursor: '#000000'
  cursor_line: '#000000'
  cursor_line_gutter: '#000000'
  matched_bracket: '#000000'
  selection: '#000000'

syntax:
  json_key: '#000000'
  json_string: '#000000'
  json_number: '#000000'
  json_boolean: '#000000'
  json_null: '#000000'

url:
  base: '#000000'
  protocol: '#000000'
  separator: '#000000'

variable:
  resolved: '#000000'
  unresolved: '#000000'

method:
  get: '#000000'
  post: '#000000'
  put: '#000000'
  delete: '#000000'
  patch: '#000000'
  options: '#000000'
  head: '#000000'

we should be able to achieve an entirely black screen, but the best I can do at the moment is