feat: adopt the native OS theme (#66)

Reviewed-on: reya/coop#66

assets/icons/git-pull-request-draft.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none"><circle cx="6.5" cy="6" r="2.25" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/><circle cx="6.5" cy="18" r="2.25" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/><circle cx="17.5" cy="18" r="2.25" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/><path d="M6.5 8.25V15.75" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/><path d="M18.25 6C18.25 6.41421 17.9142 6.75 17.5 6.75C17.0858 6.75 16.75 6.41421 16.75 6C16.75 5.58579 17.0858 5.25 17.5 5.25C17.9142 5.25 18.25 5.58579 18.25 6Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/><path d="M18.25 11.25C18.25 11.6642 17.9142 12 17.5 12C17.0858 12 16.75 11.6642 16.75 11.25C16.75 10.8358 17.0858 10.5 17.5 10.5C17.9142 10.5 18.25 10.8358 18.25 11.25Z" stroke="black" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/></svg>