palemoon theme fix

Pale Moon doesn't like specifying an "alternate stylesheet" without a title (and
specifying title creates more problems), so just set rel to an empty string.
Seems to work in Pale Moon and chromium, but I think this should be generally ok

https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel

> In this case, on <link> and <form>, if the rel attribute is absent, has no
> keywords, or if not one or more of the space-separated keywords above, then
> the element does not create any links.

gulp/res/js/theme.js

@@ -59,7 +59,7 @@ function changeTheme(type) {
 				if (css) {
 					tempLink.innerHTML = css; //set as inline style temporarily
 				}
-				defaultLink.rel = 'alternate stylesheet'; //disable default theme
+				defaultLink.rel = ''; //disable default theme
 				const themeLink = document.createElement('link');
 				themeLink.rel = 'stylesheet';
 				themeLink.id = `custom${type}`;