Fixes #12674

* The problem with #12674 is that it reassigns the same variable, but the color values were purposely different.
* This uses Less's color functions to generate the proper darker colors.
* Why darker colors? Because those CSS carets render lighter than their solid border counterparts. Consider it a design hack.

less/variables.less

@@ -525,9 +525,9 @@
 //** Popover outer arrow width
 @popover-arrow-outer-width:           (@popover-arrow-width + 1);
 //** Popover outer arrow color
-@popover-arrow-outer-color:           rgba(0,0,0,.25);
+@popover-arrow-outer-color:           fadein(@popover-border-color, 5%);
 //** Popover outer arrow fallback color
-@popover-arrow-outer-fallback-color:  #999;
+@popover-arrow-outer-fallback-color:  darken(@popover-fallback-border-color, 20%);
 
 
 //== Labels