Este tema não funciona no Navegador Firefox versão 68.9.0 móvel no Android 4.1.1 do tablet Rockchip DL 1603. / This theme does not work on Firefox Browser version 68.9.0 mobile on Android 4.1.1 of the Rockchip DL 1603 tablet.
I've confirmed this issue. I'll see what I can do to fix this.
Update: The bad news is dark theme support on Firefox Android is buggy, and may not be fixable at all, but the good news is you can have full dark mode support in Firefox Preview, which is soon to be the default Firefox app for Android. You can install Firefox Preview from play store.