Make trackbar color configurable

Use `main.trackbar` in themes.

src/config/theme.h

@@ -139,6 +139,7 @@ typedef enum {
     THEME_MAGENTA_BOLD,
     THEME_TEXT_HISTORY,
     THEME_CMD_WINS_UNREAD,
+    THEME_TRACKBAR,
 } theme_item_t;
 
 void theme_init(const char *const theme_name);