Minor graphical improvements

- Uniform style of SVGs
- Add style directly inside SVGs to switch to dark mode - the previous solution didn't work in Chromium for some reason

templates/default/assets/bitcoin.svg

@@ -1 +1 @@
-<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>
+<svg xmlns="http://www.w3.org/2000/svg" id="svg9420" width="100" height="100" version="1.1"><style>@media (prefers-color-scheme:dark){*{stroke:#fff!important}}</style><circle id="circle9414" cx="50" cy="50" r="47.503" style="fill:none;stroke:#000;stroke-width:4.99362;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"/><ellipse id="ellipse9416" cx="50" cy="50" rx="38.972" ry="39.359" style="fill:none;stroke:#000;stroke-width:5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"/><path id="path9418" d="M57.019 30.033H38.762v19.856h18.257a9.929 9.929 0 0 0 0-19.856zm0 19.856H38.762v19.857h18.257a9.929 9.929 0 0 0 0-19.857zM44.763 23.411v6.206m10.724-6.206v6.206M44.763 70.382v6.207m10.724-6.207v6.207M33.139 30.033h6.206M33.14 69.746h6.206" style="fill:none;stroke:#000;stroke-width:5.35777;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none"/></svg>