Download the trial version and try the Enlightened and Enlightened print themes.
I have personally noticed issues with using Classic in newer versions of Filemaker. They may not affect you. One thing, is that layouts (and objects created on those layouts) based on the Classic theme (even if they've been modified and re-saved as new themes) have trouble supporting newer features. For example, I couldn't get a modified Classic layout to accept a pop-up menu without drop shadow, even if I copied and pasted it from a layout with a "modern" theme
We've been running into troubles with non-classic themes under 'industrial' Windows 8.1 (only 4GB Ram, no graphics card).
Classic turnd to be the fastest, took more time to calculate - but was the fastest when it came to rendering.
It was under 14.03. Since FileMaker never tells all the changes in the release notes, customers hesitated to do the move to 14... (there was nothing said about rendering changes)
Under MacOS, we did not have that problem.