It is not possible to select and set the CSS visually for Row Left, Row Center and Row Right divs in Oxygen‘s Header Builder element since they are disabled by default.
Watch the video below to use the Selectors feature to get around this:
Here’s the summary:
In the Oxygen editor, go to Manage > Selectors.
Create a folder (optional), select it and add a new selector.
for Row Left or
for Row Center or
for Row Right.
Then you can style these using all the options that Oxygen provides.