-platform Linuxfb and bad color

[gianc]

Hi,
I use Rpi 4 with 2020-05-27-raspios-buster-armhf (raspberrypi 4.19.118-v7l+ #1311 SMP Mon Apr 27 14:26:42 BST 2020 armv7l GNU/Linux).
I wanted to develop a small qt program, a simple example with 1 label and 1 button.
If I launch the program in the graphic enviroment it is ok.
But without X enviroment (before I stop it with sudo systemctl stop lightdm) and with -platform linuxfb options, my program not uses color.
I attach 2 images.

I changed also /boot/config.txt option dtoverlay=vc4-fkms-v3d on/off
ON
mode "1920x1080"
geometry 1920 1080 1920 1080 32
timings 0 0 0 0 0 0 0
accel true
rgba 8/16,8/8,8/0,0/0
endmode

Frame buffer device information:
Name : DRM emulated
Address : 0
Size : 8294400
Type : PACKED PIXELS
Visual : TRUECOLOR
XPanStep : 1
YPanStep : 1
YWrapStep : 0
LineLength : 7680
Accelerator : No

OFF
mode "1824x984"
geometry 1824 984 1824 984 32
timings 0 0 0 0 0 0 0
rgba 8/16,8/8,8/0,8/24
endmode

Frame buffer device information:
Name : BCM2708 FB
Address : 0x3e4e0000
Size : 7179264
Type : PACKED PIXELS
Visual : TRUECOLOR
XPanStep : 1
YPanStep : 1
YWrapStep : 0
LineLength : 7296
Accelerator : No

but nothing change ...
Do you have tips?

[gianc]

I did further tests and found that:

1. the background color for widgets is black but I can change with css
2. the border are missing in new QDialog object.

a incorrect Qt configuration?

[gianc]

SOLVED:
By setting the QT_DEBUG_PLUGINS variable and launching the program it shows the list of available plugins, styles included.
The program uses, by default, the style bb10dark.
Using the parameter -style bb10bright (or cleanlooks, cde, plastique) the application has the correct colors.
Example :
./my_test_program -platform linuxfb -style cde