I think RTI might be more flexible than you think. The sliders you mention are only for our iOS and Android RTIPanel devices. On all of our controllers, you can create your own sliders, or use third-party graphics. Still, we are the midst of an RTIPanel refresh which would improve graphic selection for sliders, toggles, and gauges, which is more of a result of what we were allowed to do with these operating systems.
Integration Designer APEX has completely scalable graphics, making it simple to stretch and resize graphics, even framework. You can also easily embed icons into the bitmaps, making it a lot easier to work with. There are some things to do in your case, where you are using a background that matches the color of the RTIPanel sliders. You could easily put a shade behind the slider, making it easier to see. There are other techniques as well, as it is very flexible.
I have attached an example of something you could do with sliders on a background that matches the slider base; it is only an example, but again, this only affects RTIPanel devices and not RTI controllers. We feel the graphical capabilities of APEX is among the most flexible and simplistic in the industry.[Link: app.box.com]
If I can assist you in any way, please let me know.