There is one way to center an ROI form on an ROI Landing Page.   

Navigate to the HTML Source Editor when in your ROI Landing Page and add the following five lines of code in the appropriate places, as shown in the image below. 

Add the following code in the header.

Adding the following code around the ROI form formula, but make sure you add the correct form ID, not 414. 

Add the following code around the ROI Form formula: 

e.g. $${ROI.Forms- FORM ID}:

Below is what the form looks like without the centering code added in the HTML Editor:

Please contact with any followup questions.