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/fe_emummc_tools.c

@@ -33,8 +33,6 @@
 #define OUT_FILENAME_SZ      128
 #define NUM_SECTORS_PER_ITER 8192 // 4MB Cache.
 
-extern volatile boot_cfg_t *b_cfg;
-
 void load_emummc_cfg(emummc_cfg_t *emu_info)
 {
 	memset(emu_info, 0, sizeof(emummc_cfg_t));