tabcounter: show border in esr (MR 63)

Switching to a better suited color-variable that also works with ESR

Tweaked-by: Oliver Smith <ollieparanoid@postmarketos.org>

src/userChrome/tabcounter.css

@@ -38,7 +38,7 @@
             left: 50%;
             transform: translateX(-50%);
             /* Tab Manager Menu button styling inspired by Firefox for Android */
-            border: 1px solid var(--tab-icon-overlay-fill);
+            border: 1px solid var(--toolbarbutton-icon-fill);
             border-radius: var(--tab-border-radius);
             padding: 0 5px 0 5px;
             font-size: 10px !important;