Since the final product is rendered by the browser, there's really no "exact" render from what you see in FM vs. WD. The rendering will differ slightly anyways depending on the browser you use. Since you can't control the rendered CSS file directly for WD, then you need to make your own adjustments in FM.
My recommendation is to produce a test layout with a sampling of objects from across your database, and retheme / assess what works for you. If you invest a little time up front and decide on a theme, then you can customize that theme and apply changes to any layout using that theme across your entire file at once.
Personally, I would recommend a theme that does not use any background image or "framing" (IE any of the glass, rectangular, rounded, semi rounded or specialty themes). I think the aspire theme series works best in WD (enlightened, vibrant, sophisticated, tranquil, but not luminous due to it's bg image).
Also, a good recommendation is to separate the UI between the desktop and WD, so you can either use different themes, or adjust for any WD "quirks". If you're going to have heavy WD use, it might even be practical to separate the entire WD system out to it's own file using the data separation model.
Thanks Mike. I felt that Filemaker would ship new themes to accomodate the WebDirect. I'll test the Aspire.
Note that a separate WD system can be handled in a new file with TOs from any existing database. Kind of a semi-separation model. Interesting idea, can the GUI file be hosted next to the Data file?
The aspire themes were new for 13, so I’m sure they were more designed with WD in mind than the earlier themes. However FM was developing WD at the time 12 came out, so one has to wonder.