nyx: lvgl: complete black theme and add options

Since, this has the side-effect to make any theme color component from 0x0B to 0xC7
work, add theme background controls in Nyx Options.
Additionally, set shadow color to black and fix inactive buttons on transparent styles.

nyx/nyx_gui/config.h

@@ -45,7 +45,7 @@ typedef struct _hekate_config
 
 typedef struct _nyx_config
 {
-	u32 theme_bg;
+	u32 theme_bg; // COLOR_BG_BASE_MIN - COLOR_BG_BASE_MAX.
 	u32 theme_color;
 	u32 entries_5_col;
 	u32 timeoffset;