Use system default theme setting is invalid

[witt-bit]

This problem did not exist when I first installed guake. By chance, I switched the system from dark theme back to light theme, and guake could not detect this change and always kept the light theme.

Now no matter what I try, it can't detect it

$ guake --support                                                                                                  ✔ 
<details><summary>$ guake --support</summary>

Guake Version:		3.11.dev0

Vte Version:		0.76.4

Vte Runtime Version:	0.76.4

--------------------------------------------------
GTK+ Version:		3.24.43

GDK Backend:		<GdkX11.X11Display

--------------------------------------------------
Desktop Session: plasma

--------------------------------------------------
Display: :0

RGBA visual: True

Composited: True

* Monitor: 0 - CMN eDP-1
    * Geometry:		1920 x 1080 at 0, 0
    * Size:		309 x 173 mm²
    * Primary:		True
    * Refresh rate:	60.007 Hz
    * Subpixel layout:	unknown