[+] Separate lightness values for light and dark themes

[AnonymousRand]

Description

Add a lightness_per_mode config key accepting separate lightness values for dark and light themes.

(Note: PR also contains the same commit from #528 since I branched off my fork for that)

Relevant Links

Closes #529.

Screenshots

Before, with

"auto_detect_light_dark": true,
"lightness": 0.85

After, with

"auto_detect_light_dark": true,
"lightness_per_theme": {
  "light": 0.85,
  "dark": 0.6
}

Additional context

[hykilpikonna]

Thanks!