remove transparency from tooltips:

so that caret overlap isn't visible and so that tooltips work better
over dark as well as light backgrounds

Author: broccolini

modules/primer-support/lib/variables/misc.scss

@@ -19,7 +19,7 @@ $box-shadow-extra-large: 0 10px 50px rgba($black, 0.07) !default;
 
 // Tooltips
 $tooltip-max-width: 250px !default;
-$tooltip-background-color: rgba($black, 0.8) !default;
+$tooltip-background-color: $black;
 $tooltip-text-color: $white !default;
 $tooltip-delay: 0.4s !default;
 $tooltip-duration: 0.1s !default;