UI: Fix color of popout icon

In the light theme, the dock popout icon was the wrong color,
basically making it invisible.

UI/data/themes/Light/popout.svg

@@ -1 +1 @@
-<svg class="feather feather-plus" fill="none" stroke="#FFF" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" version="1.1" viewBox="0 0 40 40" xmlns="http://www.w3.org/2000/svg"><rect x="2" y="2" width="24" height="20" /><rect x="2" y="2" width="24" height="4" style="fill:#fff" /><rect x="14" y="18" width="24" height="20" /><rect x="14" y="18" width="24" height="4" style="fill:#fff" /></svg>
+<svg class="feather feather-plus" fill="none" stroke="#202020" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" version="1.1" viewBox="0 0 40 40" xmlns="http://www.w3.org/2000/svg"><rect x="2" y="2" width="24" height="20" /><rect x="2" y="2" width="24" height="4" style="fill:#202020" /><rect x="14" y="18" width="24" height="20" /><rect x="14" y="18" width="24" height="4" style="fill:#202020" /></svg>