philomena rainbow theme chrome extension. Instructions: Download ZIP, extract, enable Developer mode on Extensions page, "Load unpacked"

manifest.json

{
  "manifest_version": 3,
  "name": "Philomena Rainbow Theme",
  "version": "1.0.0",
  "description": "Rainbow theme color for Derpibooru",
  "permissions": ["declarativeNetRequest", "webRequest"],
  "host_permissions": ["https://derpibooru.org/*"],
  "background": {
    "service_worker": "service_worker.js"
  }
}

service_worker.js

fetch("https://derpibooru.org/themes")
  .then((r) => r.json())
  .then(async (json) => {
    const entries = Object.entries(json).map(([name, path]) => [
      name,
      path.substring(0, path.indexOf("?")),
    ]);

    const darkThemePaths = entries
      .filter(([name]) => name.startsWith("dark-"))
      .map(([, path]) => path);
    const lightThemePaths = entries
      .filter(([name]) => name.startsWith("light-"))
      .map(([, path]) => path);

    await chrome.storage.session.set({
      darkThemePaths,
      lightThemePaths,
    });
  });

chrome.webRequest.onBeforeRequest.addListener(
  async () => {
    const { darkThemePaths, lightThemePaths } =
      await chrome.storage.session.get(["darkThemePaths", "lightThemePaths"]);

    if (!darkThemePaths?.length || !lightThemePaths?.length) {
      return;
    }

    const randomDarkTheme =
      darkThemePaths[Math.floor(Math.random() * darkThemePaths.length)];
    const randomLightTheme =
      lightThemePaths[Math.floor(Math.random() * lightThemePaths.length)];

    chrome.declarativeNetRequest.updateSessionRules({
      addRules: [
        {
          id: 1,
          action: {
            type: "redirect",
            redirect: {
              transform: {
                path: randomDarkTheme,
                query: "?no&vsn=d",
              },
            },
          },
          condition: {
            urlFilter: "||derpibooru.org/css/dark-*?vsn=d",
            resourceTypes: ["stylesheet"],
          },
        },
        {
          id: 2,
          action: {
            type: "redirect",
            redirect: {
              transform: {
                path: randomLightTheme,
                query: "?no&vsn=d",
              },
            },
          },
          condition: {
            urlFilter: "||derpibooru.org/css/light-*?vsn=d",
            resourceTypes: ["stylesheet"],
          },
        },
      ],
      removeRuleIds: [1, 2],
    });
  },
  {
    urls: [
      "https://derpibooru.org/css/dark-*?vsn=d",
      "https://derpibooru.org/css/light-*?vsn=d",
    ],
  },
);