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diff --git a/ext/js/app/theme-controller.js b/ext/js/app/theme-controller.js
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+/*
+ * Copyright (C) 2022  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * This class is used to control theme attributes on DOM elements.
+ */
+class ThemeController {
+    /**
+     * Creates a new instance of the class.
+     * @param {?Element} element A DOM element which theme properties are applied to.
+     */
+    constructor(element) {
+        this._element = element;
+        this._theme = 'default';
+        this._outerTheme = 'default';
+        this._siteTheme = null;
+        this._browserTheme = 'light';
+    }
+
+    /**
+     * Gets the DOM element which theme properties are applied to.
+     * @type {?Element}
+     */
+    get element() {
+        return this._element;
+    }
+
+    /**
+     * Sets the DOM element which theme properties are applied to.
+     * @param {?Element} value The DOM element to assign.
+     */
+    set element(value) {
+        this._element = value;
+    }
+
+    /**
+     * Gets the main theme for the content.
+     * @type {string}
+     */
+    get theme() {
+        return this._theme;
+    }
+
+    /**
+     * Sets the main theme for the content.
+     * @param {string} value The theme value to assign.
+     */
+    set theme(value) {
+        this._theme = value;
+    }
+
+    /**
+     * Gets the outer theme for the content.
+     * @type {string}
+     */
+    get outerTheme() {
+        return this._outerTheme;
+    }
+
+    /**
+     * Sets the outer theme for the content.
+     * @param {string} value The outer theme value to assign.
+     */
+    set outerTheme(value) {
+        this._outerTheme = value;
+    }
+
+    /**
+     * Gets the override value for the site theme.
+     * If this value is `null`, the computed value will be used.
+     * @type {?string}
+     */
+    get siteTheme() {
+        return this._siteTheme;
+    }
+
+    /**
+     * Sets the override value for the site theme.
+     * If this value is `null`, the computed value will be used.
+     * @param {?string} value The site theme value to assign.
+     */
+    set siteTheme(value) {
+        this._siteTheme = value;
+    }
+
+    /**
+     * Gets the browser's preferred color theme.
+     * The value can be either 'light' or 'dark'.
+     * @type {?string}
+     */
+    get browserTheme() {
+        return this._browserTheme;
+    }
+
+    /**
+     * Prepares the instance for use and applies the theme settings.
+     */
+    prepare() {
+        const mediaQueryList = window.matchMedia('(prefers-color-scheme: dark)');
+        mediaQueryList.addEventListener('change', this._onPrefersColorSchemeDarkChange.bind(this));
+        this._onPrefersColorSchemeDarkChange(mediaQueryList);
+    }
+
+    /**
+     * Updates the theme attributes on the target element.
+     * If the site theme value isn't overridden, the current site theme is recomputed.
+     */
+    updateTheme() {
+        if (this._element === null) { return; }
+        const computedSiteTheme = this._siteTheme !== null ? this._siteTheme : this.computeSiteTheme();
+        const data = this._element.dataset;
+        data.theme = this._resolveThemeValue(this._theme, computedSiteTheme);
+        data.outerTheme = this._resolveThemeValue(this._outerTheme, computedSiteTheme);
+        data.siteTheme = computedSiteTheme;
+        data.browserTheme = this._browserTheme;
+        data.themeRaw = this._theme;
+        data.outerThemeRaw = this._outerTheme;
+    }
+
+    /**
+     * Computes the current site theme based on the background color.
+     * @returns {'light'|'dark'} The theme of the site.
+     */
+    computeSiteTheme() {
+        const color = [255, 255, 255];
+        const {documentElement, body} = document;
+        if (documentElement !== null) {
+            this._addColor(color, window.getComputedStyle(documentElement).backgroundColor);
+        }
+        if (body !== null) {
+            this._addColor(color, window.getComputedStyle(body).backgroundColor);
+        }
+        const dark = (color[0] < 128 && color[1] < 128 && color[2] < 128);
+        return dark ? 'dark' : 'light';
+    }
+
+    /**
+     * Event handler for when the preferred browser theme changes.
+     * @param {MediaQueryList|MediaQueryListEvent} detail The object containing event details.
+     * @param {boolean} detail.matches The object containing event details.
+     */
+    _onPrefersColorSchemeDarkChange({matches}) {
+        this._browserTheme = (matches ? 'dark' : 'light');
+        this.updateTheme();
+    }
+
+    /**
+     * Resolves a settings theme value to the actual value which should be used.
+     * @param {string} theme The theme value to resolve.
+     * @param {string} computedSiteTheme The computed site theme value to use for when the theme value is `'auto'`.
+     * @returns {string} The resolved theme value.
+     */
+    _resolveThemeValue(theme, computedSiteTheme) {
+        switch (theme) {
+            case 'auto': return computedSiteTheme;
+            case 'browser': return this._browserTheme;
+            default: return theme;
+        }
+    }
+
+    /**
+     * Adds the value of a CSS color to an accumulation target.
+     * @param {[number, number, number]} target The target color buffer to accumulate into.
+     * @param {string|*} cssColor The CSS color value to add to the target. If this value is not a string,
+     *   the target will not be modified.
+     */
+    _addColor(target, cssColor) {
+        if (typeof cssColor !== 'string') { return; }
+
+        const color = this._getColorInfo(cssColor);
+        if (color === null) { return; }
+
+        const a = color[3];
+        if (a <= 0.0) { return; }
+
+        const aInv = 1.0 - a;
+        for (let i = 0; i < 3; ++i) {
+            target[i] = target[i] * aInv + color[i] * a;
+        }
+    }
+
+    /**
+     * Decomposes a CSS color string into its RGBA values.
+     * @param {string} cssColor The color value to decompose. This value is expected to be in the form RGB(r, g, b) or RGBA(r, g, b, a).
+     * @returns {?[number, number, number, number]} The color and alpha values. The color component values range from [0, 255], and the alpha ranges from [0, 1].
+     */
+    _getColorInfo(cssColor) {
+        const m = /^\s*rgba?\s*\(\s*(\d+)\s*,\s*(\d+)\s*,\s*(\d+)\s*(?:,\s*([\d.]+)\s*)?\)\s*$/.exec(cssColor);
+        if (m === null) { return null; }
+
+        const m4 = m[4];
+        return [
+            Number.parseInt(m[1], 10),
+            Number.parseInt(m[2], 10),
+            Number.parseInt(m[3], 10),
+            m4 ? Math.max(0.0, Math.min(1.0, Number.parseFloat(m4))) : 1.0
+        ];
+    }
+}