Changing the blue border colour for pinned sites in home/new tab page

You can change it like this with userContent.css:

@-moz-document url("about:newtab"), url("about:home"){
  :root{
    --newtab-primary-action-background: #f00 !important;
  }
}

That way you can specify a color for them, but setting the value to AccentColor may or may not resolve to the actual system accent color.