[dunhill]

If you look at a calibre.palette file, you'll see it contains the two modes that calibre accepts: dark and light.

{
  "dark": {
    "palette": {
      "Window": "#1e1e1e",
      "WindowText": "#d4d4d4",
      "WindowText-disabled": "#5a5a5a",
      "Base": "#252526",
      "AlternateBase": "#2d2d2d",
      "ToolTipBase": "#333333",
      "ToolTipText": "#d4d4d4",
      "ToolTipText-disabled": "#5a5a5a",
      "Text": "#d4d4d4",
      "Text-disabled": "#5a5a5a",
      "Button": "#2d2d2d",
      "ButtonText": "#d4d4d4",
      "ButtonText-disabled": "#5a5a5a",
      "BrightText": "#ffffff",
      "BrightText-disabled": "#7a7a7a",
      "Highlight": "#007acc",
      "HighlightedText": "#ffffff",
      "HighlightedText-disabled": "#7a7a7a",
      "Link": "#3794ff",
      "LinkVisited": "#9cdcfe",
      "PlaceholderText": "#808080",
      "PlaceholderText-disabled": "#5a5a5a",
      "Foreground": "#d4d4d4",
      "Foreground-disabled": "#5a5a5a",
      "Background": "#1e1e1e",
      "Background-disabled": "#2d2d2d",
      "Shadow": "#000000",
      "Shadow-disabled": "#1a1a1a",
      "Light": "#3c3c3c",
      "Light-disabled": "#2a2a2a",
      "Midlight": "#2e2e2e",
      "Midlight-disabled": "#2a2a2a",
      "Dark": "#1a1a1a",
      "Dark-disabled": "#1a1a1a",
      "Mid": "#2a2a2a",
      "Mid-disabled": "#2a2a2a"
    },
    "use_custom": true
  },
  "light": {
    "palette": {
      "AlternateBase": "#C9B194",
      "Base": "#DBDBDB",
      "BrightText": "#706D54",
      "BrightText-disabled": "#A08963",
      "Button": "#C9B194",
      "ButtonText": "#706D54",
      "ButtonText-disabled": "#A08963",
      "Highlight": "#706D54",
      "HighlightedText": "#C9B194",
      "HighlightedText-disabled": "#A08963",
      "Link": "#706D54",
      "LinkVisited": "#A08963",
      "PlaceholderText": "#706D54",
      "PlaceholderText-disabled": "#C9B194",
      "Text": "#706D54",
      "Text-disabled": "#A08963",
      "ToolTipBase": "#DBDBDB",
      "ToolTipText": "#706D54",
      "ToolTipText-disabled": "#A08963",
      "Window": "#DBDBDB",
      "WindowText": "#706D54",
      "WindowText-disabled": "#A08963",
      "Foreground": "#706D54",
      "Foreground-disabled": "#A08963",
      "Background": "#DBDBDB",
      "Background-disabled": "#C9B194",
      "Shadow": "#BCA98A",
      "Shadow-disabled": "#C9B194",
      "Light": "#EDE8E2",
      "Light-disabled": "#D6CFC4",
      "Midlight": "#E0D8CC",
      "Midlight-disabled": "#C9B194",
      "Dark": "#A08963",
      "Dark-disabled": "#C9B194",
      "Mid": "#BCA98A",
      "Mid-disabled": "#C9B194"
    },
    "use_custom": true
  }
}

But for them to have an effect on the interface, you have to select which mode you want to apply them to. You can do this in the Preferences / L&F / Color Adjustment interface.

With the plugin, everything is in the same window: import the color palette file, select the mode you want to use the palette in, and the plugin will tell you which mode calibre is currently using.

By moving the cursor with the arrows, you can see how the colors in the calibre interface change. Double-clicking applies the changes, or you can click the Apply button.