Clean up handling of dialog button spacing

packages/application/style/buttons.css

@@ -28,7 +28,6 @@ button.jp-mod-styled.jp-mod-accept {
 
 button.jp-mod-styled.jp-mod-reject {
     background: var(--md-grey-500);
-    margin-right: 12px;
     border: 1px solid var(--md-grey-500);
     color: white;
 }
@@ -62,4 +61,4 @@ button.jp-mod-styled.jp-mod-warn {
     border: none;
     box-shadow: none;
     background-color: rgba(153,153,153,.2);
-}
+}

packages/apputils/style/dialog.css

@@ -99,3 +99,8 @@
   line-height: 1.4;
   color: var(--jp-ui-font-color0);
 }
+
+
+.jp-Dialog-button.jp-mod-styled:not(:last-child) {
+  margin-right: 12px;
+}