Broken gradient color values for #mainmenu when changing the base skin

Steps:

• Go to Administration. Current skin is XWiki.DefaultSkin
  • Homepage looks like baseColibri.png
• Go to XWiki.DefaultSkin and change the Base Skin from 'colibri' to 'flamingo'
  • Homepage looks like Flamingo
• Use the skin parameter '?skin=colibri' and refresh browser's cache
  • Homepage looks like baseFlamingo.png
  • The #mainmenu gradient colors are broken

before

#mainmenu {
  background-image: linear-gradient(to bottom, #3A8ABC 0%, #0272A6 50%);
}

after

#mainmenu {
  background-image: linear-gradient(to bottom, #3A8ABC 0%, #222 50%);
}