Load light theme prior to HTML export to ensure it is present (#7643)

src/utils/exportUtils/exportCSS.ts

@@ -14,8 +14,6 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-/* eslint-disable max-len, camelcase */
-
 import customCSS from "!!raw-loader!./exportCustomCSS.css";
 
 const cssSelectorTextClassesRegex = /\.[\w-]+/g;
@@ -34,15 +32,36 @@ function mutateCssText(css: string): string {
         );
 }
 
+function isLightTheme(sheet: CSSStyleSheet): boolean {
+    return (<HTMLStyleElement>sheet.ownerNode).dataset.mxTheme?.toLowerCase() === "light";
+}
+
+async function getRulesFromCssFile(path: string): Promise<CSSStyleSheet> {
+    const doc = document.implementation.createHTMLDocument("");
+    const styleElement = document.createElement("style");
+
+    const res = await fetch(path);
+    styleElement.textContent = await res.text();
+    // the style will only be parsed once it is added to a document
+    doc.body.appendChild(styleElement);
+
+    return styleElement.sheet;
+}
+
 // naively culls unused css rules based on which classes are present in the html,
 // doesn't cull rules which won't apply due to the full selector not matching but gets rid of a LOT of cruft anyway.
 const getExportCSS = async (usedClasses: Set<string>): Promise<string> => {
     // only include bundle.css and the data-mx-theme=light styling
     const stylesheets = Array.from(document.styleSheets).filter(s => {
-        return s.href?.endsWith("bundle.css") ||
-            (s.ownerNode as HTMLStyleElement).dataset.mxTheme?.toLowerCase() === "light";
+        return s.href?.endsWith("bundle.css") || isLightTheme(s);
     });
 
+    // If the light theme isn't loaded we will have to fetch & parse it manually
+    if (!stylesheets.some(isLightTheme)) {
+        const href = document.querySelector<HTMLLinkElement>('link[rel="stylesheet"][href$="theme-light.css"]').href;
+        stylesheets.push(await getRulesFromCssFile(href));
+    }
+
     let css = "";
     for (const stylesheet of stylesheets) {
         for (const rule of stylesheet.cssRules) {