You may be missing a setting, but it's hard to see from here. It does seem strange that FileMaker 13 doesn't have the same option as 12 to remove all styles. You might try creating a separate classic themed layout, put a slider on it and then copy/paste it from there to the layout where you want this slider.
No not missing a setting, unless its well hidden!
Your solution works a treat, will take me longer than I first thought after reading your post.
Although coping and pasting works just fine with the results I am looking, I thought I could avoid object re-naming and rebuilding a lot of my layouts.
I have found that this workaround does not work - copy a classic themed object - paste to layout - copy the objects style - paste style to relevant objects.
Once you have one such object added to your layout, you might try using it to define a new style on the styles tab. I haven't tried this yet myself (need to play with that new feature at some point.) But that might allow you to select multiple objects and apply this new style to them.