Dark Theme support

Now when prefers-color-scheme is set to "dark", a dark theme will be used.
SVGs have been updated to allow for it, and also compressed to optimize.

templates/default/assets/bitcoin.svg

@@ -1,66 +1 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<svg
-   xmlns:dc="http://purl.org/dc/elements/1.1/"
-   xmlns:cc="http://creativecommons.org/ns#"
-   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-   xmlns:svg="http://www.w3.org/2000/svg"
-   xmlns="http://www.w3.org/2000/svg"
-   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
-   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
-   width="140"
-   height="141.347"
-   version="1.1"
-   id="svg10"
-   sodipodi:docname="bitcoin.svg"
-   inkscape:version="1.0.2 (e86c870879, 2021-01-15)">
-  <metadata
-     id="metadata16">
-    <rdf:RDF>
-      <cc:Work
-         rdf:about="">
-        <dc:format>image/svg+xml</dc:format>
-        <dc:type
-           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
-      </cc:Work>
-    </rdf:RDF>
-  </metadata>
-  <defs
-     id="defs14" />
-  <sodipodi:namedview
-     pagecolor="#ffffff"
-     bordercolor="#666666"
-     borderopacity="1"
-     objecttolerance="10"
-     gridtolerance="10"
-     guidetolerance="10"
-     inkscape:pageopacity="0"
-     inkscape:pageshadow="2"
-     inkscape:window-width="1859"
-     inkscape:window-height="1017"
-     id="namedview12"
-     showgrid="false"
-     inkscape:zoom="2.1411258"
-     inkscape:cx="159.77411"
-     inkscape:cy="159.841"
-     inkscape:window-x="1341"
-     inkscape:window-y="27"
-     inkscape:window-maximized="1"
-     inkscape:current-layer="svg10" />
-  <circle
-     cx="70"
-     cy="70"
-     id="ellipse2"
-     style="fill:none;stroke:#000000;stroke-width:6.82112;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"
-     r="66.589439" />
-  <ellipse
-     cx="70"
-     cy="70.672997"
-     rx="54.561001"
-     ry="55.102001"
-     id="ellipse4"
-     style="fill:none;stroke:#000000;stroke-width:7;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none" />
-  <path
-     d="M 77.476,42.128 H 53.623 v 25.943 h 23.853 a 12.972,12.972 0 0 0 0,-25.943 z m 0,25.943 H 53.623 v 25.943 h 23.853 a 12.972,12.972 0 0 0 0,-25.943 z M 61.463,33.477 v 8.108 m 14.011,-8.108 v 8.108 m -14.011,53.26 v 8.109 m 14.011,-8.109 v 8.109 M 46.277,42.128 h 8.108 m -8.108,51.886 h 8.108"
-     id="path6"
-     style="fill:none;stroke:#000000;stroke-width:7;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none" />
-</svg>
+<svg xmlns="http://www.w3.org/2000/svg" width="140" height="141.347"><g id="layer" class="bitcoin-svg"><circle cx="70" cy="70" r="66.589439" style="fill:none;stroke:#000;stroke-width:6.82112;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"/><ellipse cx="70" cy="70.672997" rx="54.561001" ry="55.102001" style="fill:none;stroke:#000;stroke-width:7;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"/><path d="M 77.476,42.128 H 53.623 v 25.943 h 23.853 a 12.972,12.972 0 0 0 0,-25.943 z m 0,25.943 H 53.623 v 25.943 h 23.853 a 12.972,12.972 0 0 0 0,-25.943 z M 61.463,33.477 v 8.108 m 14.011,-8.108 v 8.108 m -14.011,53.26 v 8.109 m 14.011,-8.109 v 8.109 M 46.277,42.128 h 8.108 m -8.108,51.886 h 8.108" style="fill:none;stroke:#000;stroke-width:7;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"/></g></svg>