Make overrides optional for packaged themes

src/theme.less

@@ -27,7 +27,7 @@
 @import "@{themesFolder}/default/@{type}s/@{element}.variables";
 
 /* Packaged Theme */
-@import "@{themesFolder}/@{theme}/@{type}s/@{element}.variables";
+@import (optional) "@{themesFolder}/@{theme}/@{type}s/@{element}.variables";
 
 /* Site Theme */
 @import (optional) "@{siteFolder}/@{type}s/@{element}.variables";
@@ -50,6 +50,6 @@
 -------------------*/
 
 .loadUIOverrides() {
-  @import "@{themesFolder}/@{theme}/@{type}s/@{element}.overrides";
+  @import (optional) "@{themesFolder}/@{theme}/@{type}s/@{element}.overrides";
   @import (optional) "@{siteFolder}/@{type}s/@{element}.overrides";
 }