dark-mode: Refactor top-of-messages logo to have CSS styles.

This changes the fill and stroke styles to be in CSS rather than
inlined in the SVG, making it easier to change and override elsewhere.

static/styles/zulip.css

@@ -99,6 +99,16 @@ p.n-margin {
     height: 24px;
 }
 
+.top-messages-logo svg circle {
+    fill: #444;
+    stroke: #444;
+}
+
+.top-messages-logo svg path {
+    fill: #fff;
+    stroke: #fff;
+}
+
 #unmute_muted_topic_notification {
     display: none;
     position: absolute;