fix: three button styles to tertiary (#15978)

* changed margin right on warning icon to 8px

* fixed to grid units from pixels

* Changed three buttons styles to tertiary

superset-frontend/src/datasource/DatasourceEditor.jsx

@@ -1078,7 +1078,7 @@ class DatasourceEditor extends React.PureComponent {
                 <span className="m-t-10 m-r-10">
                   <Button
                     buttonSize="small"
-                    buttonStyle="primary"
+                    buttonStyle="tertiary"
                     onClick={this.syncMetadata}
                     className="sync-from-source"
                   >