fix(gtk320: functions: _text_shadow_color): fix divizion by zero caus…

…ed by bg #000000 and also mistake in the formula (fixes #93)

src/gtk-3.20/scss/_functions.scss

@@ -138,7 +138,7 @@ $modules: () !default;
     $_lbg: lightness($bg) / 100%;
 
     @if lightness($tc) < 50% {
-        @return transparentize($white, 1 - $_lbg / ($_lbg * 1.3));
+        @return transparentize($white, $_lbg * .3);
     } @else {
         @return transparentize($black, $_lbg * .8);
     }