Add basic dark & light theme (#18)

gotify_tray/gui/themes/light_purple/status_active.svg

@@ -0,0 +1,49 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   width="29.348576mm"
+   height="29.348576mm"
+   viewBox="0 0 29.348576 29.348576"
+   version="1.1"
+   id="svg5"
+   sodipodi:docname="status_ok.svg"
+   inkscape:version="1.1.2 (0a00cf5339, 2022-02-04, custom)"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+  <sodipodi:namedview
+     id="namedview7"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     inkscape:document-units="mm"
+     showgrid="false"
+     inkscape:zoom="0.77771465"
+     inkscape:cx="188.37243"
+     inkscape:cy="505.96964"
+     inkscape:window-width="1284"
+     inkscape:window-height="1082"
+     inkscape:window-x="1622"
+     inkscape:window-y="248"
+     inkscape:window-maximized="0"
+     inkscape:current-layer="layer1" />
+  <defs
+     id="defs2" />
+  <g
+     inkscape:label="Layer 1"
+     inkscape:groupmode="layer"
+     id="layer1"
+     transform="translate(-24.619528,-14.753546)">
+    <circle
+       style="fill:#00db00;fill-opacity:1;stroke-width:0.264583"
+       id="path846"
+       cx="39.293816"
+       cy="29.427834"
+       r="14.674288" />
+  </g>
+</svg>