Add custom themes

[Just-Insane]

Description

Fixes #50 - Pulled from Just-Insane#10 by @EnchantedGoldenApple

Add support for user-provided custom themes via YAML file upload. Custom themes are parsed at runtime, validated against the full folds color contract, and injected as dynamic <style> tags. They persist in localStorage and appear in existing theme selectors alongside the built-in themes.

YAML example:

name: "Custom OLED Theme"
kind: dark
colors:
  Background:
    Container: "#000000"
    ContainerHover: "#0D0D0D"
    ContainerActive: "#141414"
    ContainerLine: "#1F1F1F"
    OnContainer: "#F5F5F5"

  Surface:
    Container: "#0A0A0A"
    ContainerHover: "#141414"
    ContainerActive: "#1C1C1C"
    ContainerLine: "#262626"
    OnContainer: "#F2F2F2"

  SurfaceVariant:
    Container: "#121212"
    ContainerHover: "#1C1C1C"
    ContainerActive: "#262626"
    ContainerLine: "#303030"
    OnContainer: "#F2F2F2"

  Primary:
    Main: "#C6BFFF"
    MainHover: "#BDB6FF"
    MainActive: "#B4ADFF"
    MainLine: "#AAA3FF"
    OnMain: "#1A1730"
    Container: "#2A2550"
    ContainerHover: "#312C5C"
    ContainerActive: "#383368"
    ContainerLine: "#3F3A74"
    OnContainer: "#E9E7FF"

  Secondary:
    Main: "#FFFFFF"
    MainHover: "#EAEAEA"
    MainActive: "#DCDCDC"
    MainLine: "#CFCFCF"
    OnMain: "#000000"
    Container: "#1A1A1A"
    ContainerHover: "#242424"
    ContainerActive: "#2E2E2E"
    ContainerLine: "#383838"
    OnContainer: "#F2F2F2"

  Success:
    Main: "#8FF5C8"
    MainHover: "#7AF0BC"
    MainActive: "#6EEAB3"
    MainLine: "#63E3AB"
    OnMain: "#06281C"
    Container: "#0E3D2B"
    ContainerHover: "#124634"
    ContainerActive: "#164F3C"
    ContainerLine: "#1A5845"
    OnContainer: "#D6FFF0"

  Warning:
    Main: "#F0C299"
    MainHover: "#EAB785"
    MainActive: "#E5AE79"
    MainLine: "#E0A56E"
    OnMain: "#2A1808"
    Container: "#3D2812"
    ContainerHover: "#472F15"
    ContainerActive: "#503619"
    ContainerLine: "#5A3D1C"
    OnContainer: "#FFE9D6"

  Critical:
    Main: "#FF9E9E"
    MainHover: "#FF8C8C"
    MainActive: "#FF7F7F"
    MainLine: "#FF7373"
    OnMain: "#2A0E0E"
    Container: "#3D1515"
    ContainerHover: "#471919"
    ContainerActive: "#521E1E"
    ContainerLine: "#5C2222"
    OnContainer: "#FFDADA"

  Other:
    FocusRing: "rgba(255, 255, 255, 0.6)"
    Shadow: "rgba(0, 0, 0, 1)"
    Overlay: "rgba(0, 0, 0, 0.85)"

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

~ Evie