Demo mode not working correctly

[rob-petenzi]

Setup: arch linux + hyprland (tested also on gnome with same result), using Wayland

Long story short, the regreet --demo is not working properly. When I am using a custom CSS file, it fails to load some properties, such as background colors or custom margin for the different boxes. To see my changes I need to constantly log out, which defeats the purpose of the demo mode itself.
Some of the CSS is rendered correctly, for example if I set a filter for the background image, it shows both in the demo and on the login screen.
It would seem like the problem is caused by the fact that the demo uses the system set GTK theme, which overrites the custom CSS.
I also tried GTK_THEME=Adwaita regreet --demo -s path/to/css with no success on getting the colors specified in the regreet.css. There are no warnings or error relevant on this issue on the log generated by the above command.
This is my regreet.toml:

[background]
path = "/usr/share/backgrounds/regreet2.png"
fit = "Contain"

[GTK]
application_prefer_dark_theme = true
cursor_theme_name = "Bibata Modern Ice"
cursor_blink = true
font_name = "Cantarell 14"
icon_theme_name = "Adwaita"
theme_name = "Adwaita"

[commands]
reboot = [ "systemctl", "reboot" ]
poweroff = [ "systemctl", "poweroff" ]

[appearance]
greeting_msg = "Welcome!"

My regreet.css

* {
    all: unset;
}

picture {
    filter: blur(0.2rem);
}

frame.background.top {
    margin-top: 20px;
    background-color: red;
}

This is all rendered correctly on login, but when using the demo, the colors are not applied and remain the ones of the system theme.