remove theme.less and split preboot.less to mixins.less and variables.less to isolate customization from function

lib/bootstrap.less

@@ -12,9 +12,11 @@
 // CSS Reset
 @import "reset.less";
 
-// Core
-@import "preboot.less";
-@import "theme.less";
+// Core variables and mixins
+@import "variables.less"; // Modify this for custom colors, font-sizes, etc
+@import "mixins.less";
+
+// Grid system and page structure
 @import "scaffolding.less";
 
 // Styled patterns and elements