diff --git a/test/settings/watchers/ThemeWatcher-test.tsx b/test/settings/watchers/ThemeWatcher-test.tsx
new file mode 100644
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+++ b/test/settings/watchers/ThemeWatcher-test.tsx
@@ -0,0 +1,198 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import SettingsStore from '../../../src/settings/SettingsStore';
+import ThemeWatcher from '../../../src/settings/watchers/ThemeWatcher';
+import { SettingLevel } from '../../../src/settings/SettingLevel';
+
+function makeMatchMedia(values: any) {
+    class FakeMediaQueryList {
+        matches: false;
+        media: null;
+        onchange: null;
+        addListener() {}
+        removeListener() {}
+        addEventListener() {}
+        removeEventListener() {}
+        dispatchEvent() { return true; }
+
+        constructor(query: string) {
+            this.matches = values[query];
+        }
+    }
+
+    return function matchMedia(query: string) {
+        return new FakeMediaQueryList(query);
+    };
+}
+
+function makeGetValue(values: any) {
+    return function getValue<T = any>(
+        settingName: string,
+        _roomId: string = null,
+        _excludeDefault = false,
+    ): T {
+        return values[settingName];
+    };
+}
+
+function makeGetValueAt(values: any) {
+    return function getValueAt(
+        _level: SettingLevel,
+        settingName: string,
+        _roomId: string = null,
+        _explicit = false,
+        _excludeDefault = false,
+    ): any {
+        return values[settingName];
+    };
+}
+
+describe('ThemeWatcher', function() {
+    it('should choose a light theme by default', () => {
+        // Given no system settings
+        global.matchMedia = makeMatchMedia({});
+
+        // Then getEffectiveTheme returns light
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light");
+    });
+
+    it('should choose default theme if system settings are inconclusive', () => {
+        // Given no system settings but we asked to use them
+        global.matchMedia = makeMatchMedia({});
+        SettingsStore.getValue = makeGetValue({
+            "use_system_theme": true,
+            "theme": "light",
+        });
+
+        // Then getEffectiveTheme returns light
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light");
+    });
+
+    it('should choose a dark theme if that is selected', () => {
+        // Given system says light high contrast but theme is set to dark
+        global.matchMedia = makeMatchMedia({
+            "(prefers-contrast: more)": true,
+            "(prefers-color-scheme: light)": true,
+        });
+        SettingsStore.getValueAt = makeGetValueAt({ "theme": "dark" });
+
+        // Then getEffectiveTheme returns dark
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("dark");
+    });
+
+    it('should choose a light theme if that is selected', () => {
+        // Given system settings say dark high contrast but theme set to light
+        global.matchMedia = makeMatchMedia({
+            "(prefers-contrast: more)": true,
+            "(prefers-color-scheme: dark)": true,
+        });
+        SettingsStore.getValueAt = makeGetValueAt({ "theme": "light" });
+
+        // Then getEffectiveTheme returns light
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light");
+    });
+
+    it('should choose a light-high-contrast theme if that is selected', () => {
+        // Given system settings say dark and theme set to light-high-contrast
+        global.matchMedia = makeMatchMedia({ "(prefers-color-scheme: dark)": true });
+        SettingsStore.getValueAt = makeGetValueAt({ "theme": "light-high-contrast" });
+
+        // Then getEffectiveTheme returns light-high-contrast
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light-high-contrast");
+    });
+
+    it('should choose a light theme if system prefers it (via default)', () => {
+        // Given system prefers lightness, even though we did not
+        // click "Use system theme" or choose a theme explicitly
+        global.matchMedia = makeMatchMedia({ "(prefers-color-scheme: light)": true });
+        SettingsStore.getValueAt = makeGetValueAt({});
+        SettingsStore.getValue = makeGetValue({ "use_system_theme": true });
+
+        // Then getEffectiveTheme returns light
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light");
+    });
+
+    it('should choose a dark theme if system prefers it (via default)', () => {
+        // Given system prefers darkness, even though we did not
+        // click "Use system theme" or choose a theme explicitly
+        global.matchMedia = makeMatchMedia({ "(prefers-color-scheme: dark)": true });
+        SettingsStore.getValueAt = makeGetValueAt({});
+        SettingsStore.getValue = makeGetValue({ "use_system_theme": true });
+
+        // Then getEffectiveTheme returns dark
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("dark");
+    });
+
+    it('should choose a light theme if system prefers it (explicit)', () => {
+        // Given system prefers lightness
+        global.matchMedia = makeMatchMedia({ "(prefers-color-scheme: light)": true });
+        SettingsStore.getValueAt = makeGetValueAt({ "use_system_theme": true });
+        SettingsStore.getValue = makeGetValue({ "use_system_theme": true });
+
+        // Then getEffectiveTheme returns light
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light");
+    });
+
+    it('should choose a dark theme if system prefers it (explicit)', () => {
+        // Given system prefers darkness
+        global.matchMedia = makeMatchMedia({ "(prefers-color-scheme: dark)": true });
+        SettingsStore.getValueAt = makeGetValueAt({ "use_system_theme": true });
+        SettingsStore.getValue = makeGetValue({ "use_system_theme": true });
+
+        // Then getEffectiveTheme returns dark
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("dark");
+    });
+
+    it('should choose a high-contrast theme if system prefers it', () => {
+        // Given system prefers high contrast and light
+        global.matchMedia = makeMatchMedia({
+            "(prefers-contrast: more)": true,
+            "(prefers-color-scheme: light)": true,
+        });
+        SettingsStore.getValueAt = makeGetValueAt({ "use_system_theme": true });
+        SettingsStore.getValue = makeGetValue({ "use_system_theme": true });
+
+        // Then getEffectiveTheme returns light-high-contrast
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("light-high-contrast");
+    });
+
+    it('should not choose a high-contrast theme if not available', () => {
+        // Given system prefers high contrast and dark, but we don't (yet)
+        // have a high-contrast dark theme
+        global.matchMedia = makeMatchMedia({
+            "(prefers-contrast: more)": true,
+            "(prefers-color-scheme: dark)": true,
+        });
+        SettingsStore.getValueAt = makeGetValueAt({ "use_system_theme": true });
+        SettingsStore.getValue = makeGetValue({ "use_system_theme": true });
+
+        // Then getEffectiveTheme returns dark
+        const themeWatcher = new ThemeWatcher();
+        expect(themeWatcher.getEffectiveTheme()).toBe("dark");
+    });
+});
+