Reverts changes made to header on 10 Feb 2017.

fix-github.css

/*

Apply these rules to `github.com` via the "Stylish" Google Chrome extension:
https://chrome.google.com/webstore/detail/stylish-custom-themes-for/fjnbnpbmkenffdnngjfgmeleoegfcffe?hl=en

*/
div.header.header-dark {
  background-color: rgb(245, 245, 245);   
}

div.header.header-dark .header-nav-link,
svg.octicon {
  color: rgb(51, 51, 51);   
}

div.header.header-dark .header-nav-link:hover {
  color: rgb(51, 51, 51);
  text-decoration: underline;
}

div.header.header-dark .header-search-input {
  background-color: white;
  border: 1px solid #d8d8d8;
  color: rgb(51, 51, 51);
}

div.header.header-dark .header-search-input:focus {
  border-color: #51a7e8;
  box-shadow: inset 0 1px 2px rgba(0,0,0,0.075), 0 0 5px rgba(81,167,232,0.5);
  outline: none;
}