ChatNavBar [nfc]: Use a variable instead of 'transparent'.

This is pretty clearly what was intended. Soon, though, we'll
simplify things by removing the variable -- at this point, it's just
being used as a sentinel value [1].

[1] zulip#4443 (comment)

src/nav/ChatNavBar.js

@@ -38,7 +38,7 @@ export default function ChatNavBar(props: Props) {
       edges={['top', 'right', 'left']}
       style={{
         borderColor:
-          backgroundColor === 'transparent'
+          backgroundColor === DEFAULT_TITLE_BACKGROUND_COLOR
             ? 'hsla(0, 0%, 50%, 0.25)'
             : Color(backgroundColor).darken(0.1),
         borderBottomWidth: 1,