avoid potential UB when using isprint()

all the ctype.h functions' argument must be representable as an unsigned
char or as EOF, otherwise the behavior is undefined.

config.h

@@ -100,7 +100,7 @@ char *termname = "st-256color";
 unsigned int tabspaces = 4;
 
 /* bg opacity */
-float alpha = 1.0;
+float alpha = 0.8;
 
 /* Apprentice colorscheme */
 /* Terminal colors (16 first used in escape sequence) */