Add support for custom light color palette

[huchenlei]

Resolves #2155

Previous only the system light theme was treated as light theme. User cannot define custom light theme. This PR allows user define a custom light theme with extra flag light_theme in the color palette json.

Example light_red_fg.json:

{
  "id": "light-red-fg",
  "name": "Light (Red fg)",
  "light_theme": true,
  "colors": {
    "node_slot": {},
    "litegraph_base": {},
    "comfy_base": {
      "fg-color": "#ff0000"
    }
  }
}

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[Poulpii]

Thanks to you @huchenlei.
This new feature will work on next released ?