Added black default color for popup buttons

On Linux Firefox with dark GTK theme, the three rightmost toolbar icons of the popup can be light gray, or even white some time, this fixes this issue.

src/css/popup.css

@@ -104,6 +104,7 @@ body .toolbar button {
     padding: 4px;
     font: inherit;
     background-color: white;
+    color: black;
     opacity: 0.9;
     cursor: pointer;
     vertical-align: top;