General : Add dark mode.

CSS     : Add dark mode;
Python  : Custom QR code colors;
SVG     : Add graphics for dark mode;
SVG     : Optimize vector code;
XHTML   : Modify for dark mode.

img/background.svg

@@ -1,45 +1 @@
-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<!-- Created with Inkscape (http://www.inkscape.org/) -->
-
-<svg
-   version="1.1"
-   id="svg1"
-   width="80"
-   height="92"
-   viewBox="0 0 80 92"
-   sodipodi:docname="background_0004.svg"
-   inkscape:version="1.3.2 (091e20ef0f, 2023-11-25, 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">
-  <defs
-     id="defs1" />
-  <sodipodi:namedview
-     id="namedview1"
-     pagecolor="#ffffff"
-     bordercolor="#666666"
-     borderopacity="1.0"
-     inkscape:showpageshadow="2"
-     inkscape:pageopacity="0.0"
-     inkscape:pagecheckerboard="0"
-     inkscape:deskcolor="#d1d1d1"
-     inkscape:zoom="5.9"
-     inkscape:cx="39.830508"
-     inkscape:cy="47.033898"
-     inkscape:window-width="1916"
-     inkscape:window-height="1032"
-     inkscape:window-x="0"
-     inkscape:window-y="22"
-     inkscape:window-maximized="1"
-     inkscape:current-layer="g1" />
-  <g
-     inkscape:groupmode="layer"
-     inkscape:label="Image"
-     id="g1">
-    <path
-       style="fill:none;stroke:#dadada"
-       d="M 7.9933697,3.8536538e-4 C 8.0070897,20.71715 22.520416,14.85765 32.522174,28.28434 c 3.71128,4.98214 2.20517,13.84718 2.13272,19.71605 C 34.426664,66.48723 19.361635,62.39385 10.131229,73.8543 6.4409207,78.43618 7.9933697,86.63903 7.9933697,92.0004 M 71.993374,3.8536538e-4 C 71.978074,19.85167 56.686694,14.9665 47.464564,28.1403 c -4.0466,5.78057 -2.40551,17.09222 -2.11214,23.86009 0.62031,14.31026 16.82256,13.46918 24.50308,22.18312 3.72915,4.23091 2.13787,12.56287 2.13787,17.81689"
-       id="path1" />
-  </g>
-</svg>
+<svg width="80" height="92" xmlns="http://www.w3.org/2000/svg"><path style="fill:none;stroke:#dadada" d="M8 0c0 20.72 14.52 14.86 24.52 28.28 3.71 4.99 2.2 13.85 2.13 19.72-.22 18.49-15.29 14.4-24.52 25.85C6.44 78.44 8 86.64 8 92M72 0c-.01 19.85-15.3 14.97-24.53 28.14-4.04 5.78-2.4 17.1-2.1 23.86.61 14.31 16.81 13.47 24.5 22.18C73.59 78.41 72 86.75 72 92"/></svg>