Fix #280 CSS transparent is not working on MOW

`Normal` highlight should control background color of MOW, so I removed implemented CSS `background-color: white;` .

Author: kmorihiro

common/skin/liberator.css

@@ -198,7 +198,6 @@
     #liberator-multiline-output,
     #liberator-multiline-input {
         overflow: hidden;
-        background-color: white;
         color: black;
     }