diff --git a/src/theme.js b/src/theme.js
index fa7e3f783b..92bf03ef0a 100644
--- a/src/theme.js
+++ b/src/theme.js
@@ -127,28 +127,14 @@ function getCustomTheme(themeName) {
     return customTheme;
 }
 
-/**
- * Gets the underlying theme name for the given theme. This is usually the theme or
- * CSS resource that the theme relies upon to load.
- * @param {string} theme The theme name to get the base of.
- * @returns {string} The base theme (typically "light" or "dark").
- */
-export function getBaseTheme(theme) {
-    if (!theme) return "light";
-    if (theme.startsWith("custom-")) {
-        const customTheme = getCustomTheme(theme.substr(7));
-        return customTheme.is_dark ? "dark-custom" : "light-custom";
-    }
-
-    return theme; // it's probably a base theme
-}
-
 /**
  * Called whenever someone changes the theme
+ * Async function that returns once the theme has been set
+ * (ie. the CSS has been loaded)
  *
  * @param {string} theme new theme
  */
-export function setTheme(theme) {
+export async function setTheme(theme) {
     if (!theme) {
         const themeWatcher = new ThemeWatcher();
         theme = themeWatcher.getEffectiveTheme();
@@ -190,38 +176,41 @@ export function setTheme(theme) {
 
     styleElements[stylesheetName].disabled = false;
 
-    const switchTheme = function() {
-        // we re-enable our theme here just in case we raced with another
-        // theme set request as per https://github.com/vector-im/riot-web/issues/5601.
-        // We could alternatively lock or similar to stop the race, but
-        // this is probably good enough for now.
-        styleElements[stylesheetName].disabled = false;
-        Object.values(styleElements).forEach((a) => {
-            if (a == styleElements[stylesheetName]) return;
-            a.disabled = true;
-        });
-        Tinter.setTheme(theme);
-    };
+    return new Promise((resolve) => {
+        const switchTheme = function() {
+            // we re-enable our theme here just in case we raced with another
+            // theme set request as per https://github.com/vector-im/riot-web/issues/5601.
+            // We could alternatively lock or similar to stop the race, but
+            // this is probably good enough for now.
+            styleElements[stylesheetName].disabled = false;
+            Object.values(styleElements).forEach((a) => {
+                if (a == styleElements[stylesheetName]) return;
+                a.disabled = true;
+            });
+            Tinter.setTheme(theme);
+            resolve();
+        };
 
-    // turns out that Firefox preloads the CSS for link elements with
-    // the disabled attribute, but Chrome doesn't.
+        // turns out that Firefox preloads the CSS for link elements with
+        // the disabled attribute, but Chrome doesn't.
 
-    let cssLoaded = false;
+        let cssLoaded = false;
 
-    styleElements[stylesheetName].onload = () => {
-        switchTheme();
-    };
+        styleElements[stylesheetName].onload = () => {
+            switchTheme();
+        };
 
-    for (let i = 0; i < document.styleSheets.length; i++) {
-        const ss = document.styleSheets[i];
-        if (ss && ss.href === styleElements[stylesheetName].href) {
-            cssLoaded = true;
-            break;
+        for (let i = 0; i < document.styleSheets.length; i++) {
+            const ss = document.styleSheets[i];
+            if (ss && ss.href === styleElements[stylesheetName].href) {
+                cssLoaded = true;
+                break;
+            }
         }
-    }
 
-    if (cssLoaded) {
-        styleElements[stylesheetName].onload = undefined;
-        switchTheme();
-    }
+        if (cssLoaded) {
+            styleElements[stylesheetName].onload = undefined;
+            switchTheme();
+        }
+    });
 }