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/frontend/gui_emmc_tools.c

@@ -28,8 +28,6 @@
 #include "../hos/hos.h"
 #include <libs/fatfs/ff.h>
 
-extern boot_cfg_t b_cfg;
-
 extern char *emmcsn_path_impl(char *path, char *sub_dir, char *filename, sdmmc_storage_t *storage);
 
 typedef struct _emmc_backup_buttons_t