Add border-opacity variable for improved color utility (#2061)

.changeset/four-actors-fix.md

@@ -0,0 +1,5 @@
+---
+"@tabler/core": patch
+---
+
+Add border-opacity variable for improved color utility

src/scss/utils/_colors.scss

@@ -13,6 +13,7 @@
   }
 
   .border-#{"" + $color} {
+    --#{$prefix}border-opacity: 1;
     border-color: rgba(var(--#{$prefix}#{$color}-rgb), var(--#{$prefix}border-opacity)) !important;
   }
 }