Caution: at this stage of Lubuntu's move to LXQt, you can't completely get what you want with the available defaults! But the following suggestions may offer some relief!
Lubuntu 18.10 has gtk3 applications and qt applications and uses Openbox as its window manager. These need to be addressed separately.
Window manager
Lubuntu 18.10 comes with a variety of Openbox themes: some are "dark" with a dark background and light text and window buttons.
To access the graphical user interface, click on the Menu button > Preferences > LXQt settings > Openbox settings > Theme. Move your mouse pointer to the pane on the right hand side and scroll up/down to select the theme of your choice. The appearance of the top bar dynamically changes to reflect the theme. I've chosen Lubuntu Arc. Clicking Close will enforce the selected theme.
gtk3 applications
There some "dark" themes available in the repositories. So you don't really have to resort to ppas or external downloads.
To illustrate, if you run sudo apt install blackbird-gtk-theme, that will install a dark theme suitable for both gtk2 and gtk3 applications.
To select Blackbird as your choice for gtk2/gtk3 applications, again click on Menu > Preferences > LXQt settings and then click on Appearance.
In the image below, I've chosen Blackbird for both gtk2 and gtk3 applications.
qt applications
This is a work-in-progress by the Lubuntu team.
As above, click on Menu > Preferences > LXQt settings > Appearance.
You'll notice that Qt Style does not offer a dark option at this time. Both Breeze and Fusion are "light" and Windows is probably something you don't want.
The result of using Breeze or Fusion is that the area immediately below the application's window decorations and below the main window are light (and not what you want).
LXQt panel
The same graphical user interface can be used to select from a variety of panel themes. In Menu > Preferences > Appearance, click on LXQt Theme and make your selection.
Here's what a few applications look like:
- QTerminal and Featherpad both have a dark background with light text (other than the area themed by Qt Style).
- PCManFM-Qt is completely light with dark text.
- Mousepad, a gtk3 application, is completely dark with light text.
It's quite possible that someone here knows how to get the appearance you desire but this is as far as I go!
Best Answer
The issue is a problem with the Ambiance theme. There is a highlight, but the background color for the prelight is darker. If thunderbird is showing a light highlight and inverting the colors of the icons, then it is because it's using custom widgets in XUL (which isn't Gtk+), which aren't following the GTK+ or icon themes, for everything.