Improve phrasing

webpack.config.js

@@ -25,7 +25,7 @@ const cssThemes = {
 };
 
 function getActiveThemes() {
-    // We want to use `light` theme by default if it's not defined.
+    // Default to `light` theme when the MATRIX_THEMES environment variable is not defined.
     const theme = process.env.MATRIX_THEMES ?? 'light';
     const themes = theme.split(',').filter(x => x).map(x => x.trim()).filter(x => x);
     return themes;