Fix black tab borders on Standalone (regression 53d1bca)

It appears we do not have ThreeDHighlight in Standalone.
Relevant bug https://bugzilla.mozilla.org/show_bug.cgi?id=1261068

chrome/content/zotero-platform/unix/overlay.css

@@ -200,14 +200,14 @@ toolbar:not([id="nav-bar"]) #zotero-toolbar-buttons separator {
 /* Fixes tabs missing styling on (GTK 3.20) Ubuntu 16.10. See https://bugzilla.mozilla.org/show_bug.cgi?id=1306425 */
 #zotero-pane tabpanels, #zotero-prefs tabpanels {
 	-moz-appearance: none;
-	border: 1px solid ThreeDHighlight;
+	border: 1px solid hsla(0, 0%, 0%, 0.2);
 
 }
 #zotero-pane tab, #zotero-prefs tab {
 	-moz-appearance: none;
-	border-top: 1px solid ThreeDHighlight;
-	border-right: 1px solid ThreeDHighlight;
-	border-left: 1px solid ThreeDHighlight;
+	border-top: 1px solid hsla(0, 0%, 0%, 0.2);
+	border-right: 1px solid hsla(0, 0%, 0%, 0.2);
+	border-left: 1px solid hsla(0, 0%, 0%, 0.2);
 	-moz-border-top-colors: none;
 	-moz-border-right-colors: none;
 	-moz-border-left-colors: none;