Fix #slider in Dark theme

See https://github.com/FreshRSS/FreshRSS/commit/6b560c61330b254e794057ef36515e4c751114f4
Fix https://github.com/FreshRSS/FreshRSS/issues/750

p/themes/Dark/dark.css

@@ -879,6 +879,12 @@ a.btn {
 	border-radius: 3px;
 }
 
+/*=== Slider */
+#slider {
+	background-color: #1c1c1c;
+	border-left: 1px solid #666;
+}
+
 /*=== DIVERS */
 /*===========*/
 .aside.aside_feed .nav-form input,
@@ -955,9 +961,6 @@ a.btn {
 	background: #111;
 	color: #eee;
 }
-#slider.active {
-	background-color: #1c1c1c;
-}
 
 /*=== MOBILE */
 /*===========*/