apply color white to slack notification bg color on light mode

src/client/styles/scss/theme/_apply-colors-light.scss

@@ -36,6 +36,12 @@ $table-hover-bg: $bgcolor-table-hover;
   background-color: $bgcolor-global;
 }
 
+.grw-slack-notification {
+  .form-control {
+    background: white;
+  }
+}
+
 .form-control::placeholder {
   color: darken($bgcolor-global, 20%);
 }