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.svg

@@ -1,13 +1,6 @@
 <?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">
- <style>
-  @media (prefers-color-scheme: dark) {
-   #polymc-header-text {
-    fill: white;
-   }
-  }
- </style>
  <defs>
   <linearGradient id="linearGradient84726" x1="4.4979" x2="12.435" y1="3.8011" y2="9.5681" gradientUnits="userSpaceOnUse">
    <stop stop-color="#88b858" offset="0"/>
@@ -27,7 +20,7 @@
   </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" transform="matrix(6.9306 0 0 6.9306 -702.9 -659.02)" stroke-width=".26458" aria-label="PolyMC">
+ <g id="polymc-header-text" fill="white" 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"/>