style: add ease in out on spinner background

src/sass/main.scss

@@ -128,7 +128,7 @@ button .mat-progress-spinner circle, button .mat-spinner circle {
 .spinner-background {
   background-color: rgba(255, 255, 255, 0.66);
   min-height: 100px;
-  animation: from-transparent-background-color 0.5s;
+  animation: from-transparent-background-color 0.5s ease-in-out;
 }
 
 @keyframes from-transparent-background-color {