For accessibility, we have the OWA Light client that was specifically designed with screen readers and low vision users in mind. Font scaling and high contrast color schemes also work best with the OWA Light client, which is available from the OWA Premium logon screen by selecting the "Use Outlook Web Access Light" check box. You can also select to "Use the blind and low vision experience" from the General Settings page in the OWA Premium Options area.
To create a custom theme for OWA Premium that is accessible, you could create that colorscheme yourself using the directions above. We haven't fully tested using larger fonts, but it can be done using the stylesheet for sure - you'll just have to verify the changes appear correctly.