How to use transparency in legacy Thunderbird without destroying the title bar?

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For experimental purposes I have Thunderbird v.52 x86 (pre-quantum and with legacy addons) installed on one of my computers (Windows 7 here). Now I would like some GUI elements to have a transparent background color – this is possible with CSS, whether userChrome.css or addon does not matter.
But there is an unexpected negative side effect. If I specify transparency with RGBA scheme or procentual opacity, then title bars of child windows get lost (no joke). The same goes for the about:config setting ui.use_native_colors if it is set to false:

It definitely has to do with the transparency colors. Because if I choose an normal RGB color and without opacity specifications, the color gets applied and title bar is kept available too.

My idea:

  • Keeping ui.use_native_colors = true and only override native colors on target elements like this one:
/* Symbol bar: Write new mail */
#msgcomposeWindow {
    background-color: rgba(255, 255, 255, 0.0);

What I have tried so far:

  • different approaches for transparency like background-color: rgba(255, 255, 255, 0.0);, opacity: 0.99;, color: transparent;
  • used transparent background image -> is not shown at all and default colors remain
  • set transparency to needed elements only (to avoid native colors forbidden in general)
  • tried existing addons like [email protected]
  • tried JavaScript to temporary override native color usage
  • restarted computer and Thunderbird several times as well as deleted whole TB profile to verify it’s not a one-time bug

Does anyone have an idea which changes to the CSS are necessary to keep both, transparency symbol bars and the title bar that works with my Thunderbird 52?

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