Created attachment 8880
Some pressed and some unpressed buttons in LibreOfficeWriter whti High Contrast theme
The inscription on the buttons is in a dark grey. Once a button (e.g. the one for bold) is pressed, its background is flooded black, Then the inscription is only badly visible.
On former versions of Xubuntu I used Bluebird. However, on 18.04 it is not present and after copying the two related directories to their respective plages, Bluebird did not look well: The names of the main menu items e.g. of LibreOffice: File/Edit/... were so close to each other that they even overlapped.
Then I switched to "Hoher Kontrast" (English version: High Contrast).
However in this one, LibreOffice Writer's buttons for Bold / Italics / Underline and so on were almost unreadable if they were pressed.
The reason is probably that the letters on the button are not in black but in very dark grey. If the background of such a key is painted dark, this dark inscription has almost zero contrast with its surroundings! That's not what one expects from a Hight Contrast theme.
Created attachment 8882
comparison original / inverted / proposed special inversion
Here you see some of the buttons from LO Writer, which can stay in the pressed state. The easiest thing would be the inversion, shown in the second line. However, some small details are not high contrast.
Therefore my proposal of the third line: a special inversion.
I show the inversion curve made with GIMP in my next comment's attachment.
Created attachment 8883
The bent curve is the inversion applied in the third row. This one shows the added items more clearly.
In the pressed state, a button should best be shown in such a special inversion mode. This can be done by applying a proper look up table for the pixle colors.
The window manager (xfwm4) has nothing to do with the applications drawing their own widgets.
Better report those to the relevant applications.
Closing as invalid.