Gray out button on disable

GitLab: #195

Change-Id: I6485a81186ce5b30abcdbd85742eaf5d7de42b6d
diff --git a/client/src/components/CallButtons.tsx b/client/src/components/CallButtons.tsx
index d1beab8..b77fd5a 100644
--- a/client/src/components/CallButtons.tsx
+++ b/client/src/components/CallButtons.tsx
@@ -85,6 +85,9 @@
     '&:hover': {
       backgroundColor: paletteColor.main,
     },
+    '&:disabled': {
+      backgroundColor: theme.palette.action.disabled,
+    },
   };
 });