Make site theme optional

src/theme.less

@@ -22,8 +22,8 @@
      Site Theme
 -------------------*/
 
-@import "@{siteFolder}/globals/site.variables";
-@import "@{siteFolder}/@{type}s/@{element}.variables";
+@import (optional) "@{siteFolder}/globals/site.variables";
+@import (optional) "@{siteFolder}/@{type}s/@{element}.variables";
 
 
 /*******************************
@@ -44,5 +44,5 @@
 
 .loadUIOverrides() {
   @import "@{themesFolder}/@{theme}/@{type}s/@{element}.overrides";
-  @import "@{siteFolder}/@{type}s/@{element}.overrides";
+  @import (optional) "@{siteFolder}/@{type}s/@{element}.overrides";
 }