Update plot smooth control background color (#4202)

Author: julieg18

webview/src/plots/components/styles.module.scss

@@ -185,7 +185,8 @@ $gap: 20px;
   position: absolute;
   bottom: 0;
   width: 100%;
-  background-color: $plot-block-bg-color;
+  box-shadow: inset 250px 250px $fg-transparency-1;
+  background-color: $bg-color;
   color: $fg-color;
   padding: 10px;
   display: none;