diff --git a/src/app/AppConfig.ts b/src/app/AppConfig.ts
index dd51aa21..c4d3fcc8 100644
--- a/src/app/AppConfig.ts
+++ b/src/app/AppConfig.ts
@@ -83,6 +83,11 @@ export type AppConfig = {
 	 * Default: 1.
 	 */
 	zoomFactor: number
+	/**
+	 * User-provided CSS injected into desktop app windows.
+	 * Default: empty string.
+	 */
+	customCss: string
 
 	// ----------------
 	// Privacy settings
@@ -143,6 +148,7 @@ const defaultAppConfig: AppConfig = {
 	systemTitleBar: false,
 	monochromeTrayIcon: isMac,
 	zoomFactor: 1,
+	customCss: '',
 	playSoundChat: 'respect-dnd',
 	playSoundCall: 'respect-dnd',
 	enableCallbox: 'respect-dnd',
diff --git a/src/shared/setupWebPage.js b/src/shared/setupWebPage.js
index 6c533968..55eef721 100644
--- a/src/shared/setupWebPage.js
+++ b/src/shared/setupWebPage.js
@@ -8,7 +8,7 @@ import { isRTL, register } from '@nextcloud/l10n'
 import { appData } from '../app/AppData.js'
 import { refetchAppData } from '../app/appData.service.js'
 import { TITLE_BAR_HEIGHT } from '../constants.js'
-import { initAppConfig } from './appConfig.service.ts'
+import { getAppConfigValue, initAppConfig } from './appConfig.service.ts'
 import { BUILD_CONFIG } from './build.config.ts'
 import { initGlobals } from './globals/globals.js'
 import { setupInitialState } from './initialState.service.js'
@@ -242,6 +242,23 @@ function applyHeaderHeight() {
 	document.body.style.setProperty('--header-height', `${TITLE_BAR_HEIGHT}px`)
 	document.documentElement.style.setProperty('--header-height', `${TITLE_BAR_HEIGHT}px`)
 }
+/**
+ * Apply custom user CSS from the desktop settings.
+ */
+function applyCustomCss(css = getAppConfigValue('customCss')) {
+	let style = document.getElementById('talk-desktop-custom-css')
+	if (!style) {
+		style = document.createElement('style')
+		style.id = 'talk-desktop-custom-css'
+		document.head.appendChild(style)
+	}
+	style.textContent = typeof css === 'string' ? css : ''
+	window.TALK_DESKTOP.onAppConfigChange((_event, { key, value }) => {
+		if (key === 'customCss') {
+			style.textContent = typeof value === 'string' ? value : ''
+		}
+	})
+}
 
 /**
  * Handle download links
@@ -267,6 +284,7 @@ export async function setupWebPage() {
 	initGlobals()
 	applyUserData()
 	applyHeaderHeight()
+	applyCustomCss()
 	applyAxiosInterceptors()
 	applyDownloadLinkHandler()
 
diff --git a/src/talk/renderer/Settings/DesktopSettingsSection.vue b/src/talk/renderer/Settings/DesktopSettingsSection.vue
index c4a75b38..8c405466 100644
--- a/src/talk/renderer/Settings/DesktopSettingsSection.vue
+++ b/src/talk/renderer/Settings/DesktopSettingsSection.vue
@@ -31,6 +31,7 @@ const theme = useAppConfigValue('theme')
 const systemTitleBar = useAppConfigValue('systemTitleBar')
 const monochromeTrayIcon = useAppConfigValue('monochromeTrayIcon')
 const zoomFactor = useAppConfigValue('zoomFactor')
+const customCss = useAppConfigValue('customCss')
 
 const playSoundChat = useAppConfigValue('playSoundChat')
 const playSoundCall = useAppConfigValue('playSoundCall')
@@ -80,6 +81,13 @@ const secondarySpeakerDevice = useAppConfigValue('secondarySpeakerDevice')
 
 		<UiFormGroupZoom v-model="zoomFactor" />
 
+		<NcFormGroup :label="t('talk_desktop', 'Custom CSS')">
+			<textarea
+				v-model="customCss"
+				:aria-label="t('talk_desktop', 'Custom CSS')"
+				spellcheck="false" />
+		</NcFormGroup>
+
 		<NcFormGroup :label="t('talk_desktop', 'Notifications & Sounds')">
 			<NcFormBox>
 				<UiFormBoxSelectNative v-model="playSoundChat" :label="t('talk_desktop', 'Play chat notification sound')" :options="notificationLevelOptions" />