fix(readme): header color should follow GH theme

As browser color scheme preference and GitHub theme can be different
from each other, let's follow GitHub theme instead of browser
preference.

program_info/polymc-header-black.svg

@@ -0,0 +1,31 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+<svg width="1424" height="512" version="1.1" viewBox="0 0 376.77 135.47" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+ <defs>
+  <linearGradient id="linearGradient84726" x1="4.4979" x2="12.435" y1="3.8011" y2="9.5681" gradientUnits="userSpaceOnUse">
+   <stop stop-color="#88b858" offset="0"/>
+   <stop stop-color="#72b147" offset=".5"/>
+   <stop stop-color="#5a9a30" offset="1"/>
+  </linearGradient>
+ </defs>
+ <g transform="matrix(6.95 0 0 6.9572 3.7759 1.0225)">
+  <g>
+   <path d="m3.561 16.016s0-3.5642 4.9056-3.5642c4.9069 0 4.9056 3.5642 4.9056 3.5642z" fill="#765338"/>
+   <path d="m8.4667 12.452-4.9056 3.5642-3.0319-9.3311z" fill="#b7835a"/>
+   <path d="m8.4667 12.452 7.9375-5.7669-3.0319 9.3311z" fill="#5b422d"/>
+   <path d="m8.8308 12.716-0.36417 0.26458-0.36417-0.26458c0-0.26458 0.36417-0.26458 0.36417-0.26458s0.36417 0 0.36417 0.26458z" fill="#72b147"/>
+   <path d="m8.4667 12.452s-2e-7 -5.7669 7.9375-5.7669l-0.22507 0.69269-0.91853 1.1965-0.91853 0.13819-0.91853 1.1965-0.91853 0.13819-0.91853 1.1965-0.91853 0.13819-0.91853 1.1965-0.91853 0.13819z" fill="#5a9a30"/>
+   <path d="m8.1025 12.716-0.91853-0.13819-0.91853-1.1965-0.91853-0.13819-0.91853-1.1965-0.91853-0.13819-0.91853-1.1965-0.91853-0.13819-0.91853-1.1965-0.22507-0.69269c7.9375 1e-7 7.9375 5.7669 7.9375 5.7669z" fill="#88b858"/>
+   <path d="m0.52917 6.6846 7.9375 5.7669 7.9375-5.7669-7.9375-5.7669z" fill="url(#linearGradient84726)"/>
+  </g>
+  <path d="m0.75424 7.3773-0.22507-0.69269 7.9375 5.7669 7.9375-5.7669-0.22507 0.69269-7.7124 5.6034z" fill-opacity="0"/>
+ </g>
+ <g id="polymc-header-text" fill="black" transform="matrix(6.9306 0 0 6.9306 -702.9 -659.02)" stroke-width=".26458" aria-label="PolyMC">
+  <path d="m120.51 108.48v2.7957h-1.3074v-7.5241h2.8784q1.2609 0 1.9999 0.65629 0.74414 0.65629 0.74414 1.7363 0 1.1059-0.72864 1.7208-0.72346 0.61495-2.0309 0.61495zm0-1.049h1.571q0.69763 0 1.0645-0.32556 0.3669-0.33073 0.3669-0.95084 0-0.60978-0.37207-0.97152-0.37207-0.3669-1.0232-0.37723h-1.6071z"/>
+  <path d="m125.55 108.43q0-0.82165 0.32556-1.4779 0.32556-0.66145 0.91467-1.0128 0.58911-0.35657 1.3539-0.35657 1.1317 0 1.8345 0.72864 0.70797 0.72863 0.76481 1.9327l5e-3 0.29455q0 0.82682-0.32039 1.478-0.31523 0.65112-0.9095 1.0077-0.58911 0.35657-1.3643 0.35657-1.1834 0-1.8965-0.78548-0.70796-0.79065-0.70796-2.1032zm1.2557 0.10852q0 0.863 0.35657 1.3539 0.35656 0.48576 0.99218 0.48576t0.98702-0.49609q0.35657-0.49609 0.35657-1.4521 0-0.84749-0.36691-1.3436-0.36173-0.49609-0.98701-0.49609-0.61495 0-0.97668 0.49092-0.36174 0.48576-0.36174 1.4573z"/>
+  <path d="m133.14 111.27h-1.2557v-7.9375h1.2557z"/>
+  <path d="m136.46 109.47 1.1369-3.793h1.3384l-2.2221 6.4389q-0.5116 1.4108-1.7363 1.4108-0.27388 0-0.60461-0.093v-0.97151l0.23771 0.0155q0.47542 0 0.71314-0.1757 0.24287-0.17053 0.3824-0.57877l0.18087-0.48059-1.9637-5.5655h1.3539z"/>
+  <path d="m141.48 103.75 2.1704 5.7671 2.1652-5.7671h1.6898v7.5241h-1.3022v-2.4805l0.12919-3.3176-2.2221 5.7981h-0.93534l-2.2169-5.7929 0.12919 3.3124v2.4805h-1.3022v-7.5241z"/>
+  <path d="m154.79 108.82q-0.11369 1.2041-0.88883 1.881-0.77515 0.67179-2.0619 0.67179-0.89916 0-1.5865-0.42375-0.68212-0.42891-1.0542-1.2144t-0.38758-1.8242v-0.7028q0-1.0645 0.37724-1.8758 0.37724-0.81131 1.08-1.2506 0.70797-0.43925 1.633-0.43925 1.2454 0 2.005 0.67696 0.75965 0.67696 0.88367 1.912h-1.3022q-0.093-0.81132-0.47543-1.1679-0.37723-0.36174-1.111-0.36174-0.85265 0-1.3126 0.62529-0.45475 0.62011-0.46509 1.8242v0.66662q0 1.2196 0.43408 1.8604 0.43925 0.64078 1.2816 0.64078 0.76998 0 1.1576-0.34623t0.49093-1.1524z"/>
+ </g>
+</svg>