[Night Mapping] Simplify feature dialog background is transparent

[jonnyforestGIS]

Describe the bug
with UI Theme Night Mapping active when we click on a feature to simplify the background dialogue of Simplify feature from Advanced Digitizing Toolbar disappears. Se the images with the default and night mapping themes.

How to Reproduce
With Night Mapping Them activated:

1. Pick a vector layer to edit;
2. Go to Advanced Digitizing Toolbar and click on Simplify feature button.
3. Simplify feature dialogue appears on the screen without background ( See screenshot 2)

Screenshots
Default Theme - Screenshot 1

Night Mapping Theme - Screenshot 2

QGIS and OS versions

QGIS version	3.14.16-Pi	QGIS code revision	df27394
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR 3.2.0dev	Running against GDAL/OGR	3.2.0dev
Compiled against GEOS	3.8.1-CAPI-1.13.3	Running against GEOS	3.8.1-CAPI-1.13.3
Compiled against SQLite	3.29.0	Running against SQLite	3.29.0
PostgreSQL Client Version	11.5	SpatiaLite Version	4.3.0
QWT Version	6.1.3	QScintilla2 Version	2.10.8
Compiled against PROJ	6.3.2	Running against PROJ	Rel. 6.3.2, May 1st, 2020
OS Version	Windows 10 (10.0)
Active python plugins	DataPlotly;
freehandEditing3;
Qgis2threejs;
qgis2web;
quick_map_services;
db_manager;
MetaSearch;
processing

And as well

QGIS version	3.10.10-A Coruña	QGIS code revision	4d730c3
Compiled against Qt	5.11.2	Running against Qt	5.11.2
Compiled against GDAL/OGR	3.2.0dev	Running against GDAL/OGR	3.2.0dev
Compiled against GEOS	3.8.1-CAPI-1.13.3	Running against GEOS	3.8.1-CAPI-1.13.3
Compiled against SQLite	3.29.0	Running against SQLite	3.29.0
PostgreSQL Client Version	11.5	SpatiaLite Version	4.3.0
QWT Version	6.1.3	QScintilla2 Version	2.10.8
Compiled against PROJ	7.2.0	Running against PROJ	Rel. 7.2.0, November 1st, 2020
OS Version	Windows 10 (10.0)	This copy of QGIS writes debugging output.
Active python plugins	DataPlotly;
freehandEditing3;
Qgis2threejs;
qgis2web;
quick_map_services;
db_manager;
MetaSearch;
processing

[nirvn]

Ill have a look.