The Donation Form Styles option is only available on forms where the form layout is set to standard.  You can change the form layout through these steps:
  1.  Select Fundraising > Donation Management
  2. Select All Donation Forms tab
  3. Click Edit next to your donation form
  4. Under Form Layout, select Standard
  5. Click Save

It is possible to change the styles on a responsive donation form by adding custom CSS to the form.  This can be done by selecting Design Donor Screens > Customize Look and Feel > Custom CSS box to enter this custom CSS.