Tilda custom GTK style (including a fix for the scrollbar). Do a backup of the old file, and copy this one to ~/.config/tilda/style.css

style.css

/*
  The .8 transparency value can be calculated using the transparency config value (V)
  with the formula: (100-V)/100. I have the transparency set to 20, so when I apply the
  formula, I get: (100-20)/100 = 80/100 = 0.8 = .8

  For no transparency (V = 0), set the value to 1.
*/
notebook > header {
  background-color: rgba(0,0,0,.8);
  border: none;
}
notebook > header > tabs {
  background-color: transparent;
}
notebook > header > tabs > tab {
  background: #000;
  opacity: .8;
  border-radius: 0;
  color: gray;
  border: 0;
  margin: 0 1px;
  padding: 6px;
  min-width: 100px;
}

notebook > header > tabs > tab:checked {
  color: orange;
  background-color: transparent;
}

scrollbar {
  background: #000;
  opacity: .8;
  margin: 0;
}

I've adjusted it to my colorsheme which is base16-railscast

notebook > header {
  background-color: rgba(43,43,43,.97);
  border: none;
}
notebook > header > tabs {
  background-color: transparent;
}
notebook > header > tabs > tab {
  background-color: transparent;
  opacity: .8;
  border: solid rgb(200, 200, 200);
  border-width: 0 0 0 1px;
  border-radius: 0;
  color: rgb(200, 200, 200);
  margin: 0 -1px;
  padding: 6px;
  min-width: 100px;
}

notebook > header > tabs > tab:checked {
  color: rgb(200,200,200);
  background: rgb(80,80,80);
}

scrollbar {
  background: #000;
  opacity: .8;
  margin: 0;
}