fix baground mcu image

src/actions/boardAction.js

@@ -1,9 +1,13 @@
 import {
   BOARD,
 } from "./types";
+import mini_opacity from "../data/mini_opacity.png"
+import mcu_opacity from "../data/mcu_opacity.png"
 
 export const setBoard = (board) => (dispatch) => {
   window.sessionStorage.setItem("board", board);
+  const root = document.querySelector(':root');
+  root.style.setProperty('--url', `url(${board === "mcu" ? mcu_opacity : mini_opacity})`);
   dispatch({
     type: BOARD,
     payload: board,