The two settings Workflow provides for the position of your forms is handled here:
When the checkbox is selected, your form turns out like this:
being both centered vertically as well as horizontally.
If the checkbox is unchecked, your form ends up in the corner:
In many of my forms, the UI flowed much better if the form were actually centered horizontally, but topped-out vertically.
To achieve this effect, leave this box checked:
Then, we’ll add a bit of CSS to the page by adding an IncludeHTML component. Use a Merge Text component to build the CSS ahead of the form, like so:
The guts of the Merge Text component:
And the IncludeHTML component on the form designer:
That should do it. At runtime, the form should be centered as expected.