Fix exporting from nightly where the data-mx-theme is Light not light (matrix-org#7416)

Author: t3chguy

src/utils/exportUtils/exportCSS.ts

@@ -39,7 +39,8 @@ function mutateCssText(css: string): string {
 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 === "light";
+        return s.href?.endsWith("bundle.css") ||
+            (s.ownerNode as HTMLStyleElement).dataset.mxTheme.toLowerCase() === "light";
     });
 
     let css = "";