propose a optimised DARK mode for all gadgets

[andrasrcord]

Hello, it will be interesting of propose a DARK mode, assuring than all gadgets background color will are BLACK. The goal being to avoid any ocular fatigue and glare due to the white of the current "Friture" in a music studio context. Actually, all gadgets background color remains in WHITE (except for the spectrogram gadget than him stay BLACK). Tested on Kubuntu 20.04 / 21.10 for all Plasma settings.