Put more of color-handling into JS code

src/index.css

@@ -38,7 +38,6 @@ ol, ul {
 }
 
 .square:focus {
-  background: #8d98e6;
   outline: none;
 }
 

src/index.js

@@ -70,7 +70,7 @@ function Square(props) {
   let bg = {
       backgroundImage: `url(${Images[props.value]})`,
       backgroundSize: `11vw`,
-      // backgroundColor: "lightblue",
+      backgroundColor: props.bg,
   };
   if (props.value != null) {
     return (
@@ -353,10 +353,13 @@ class Board extends React.Component {
   }
 
   renderSquare(i) {
+    let bgColor = this.heldPiece() === i ?
+      "#5D98E6" : "white";
     return (
       <Square
         value={this.state.squares[i]}
         onClick={() => this.handleClick(i)}
+        bg={bgColor}
       />
     );
   }