All SoftKeyz does is say, if there's a Back button here, use this graphic. If there's a Home button here, use this one. And so on. It's very simple. I used to use it, but I currently fancy the Android L navbar (the semi-secret one that looks normal, not the iOS-inspired shapes) so I'm using the Xposed module for that. Works wonders. Also available in SoftKeyz, but the Xposed module does it fine. Easier to do.
If you have a good AOSP ROM, there's a cherry pick that they all have access to that lets you add and remove, and sort buttons, set their long press shortcuts, and choose what they look like, all from one place. LiquidSmooth has it, but I have yet to try it on the M8. (I need my speakerphone.) It had it on the Galaxy S3, I mean. I know CM doesn't use it (they didn't support the navbar at all on the S3—not sure how they handle the M8).
tl;dr: For complete control of your navbar, you have to use LiquidSmooth, or any other AOSP ROM with that cherry pick, but then you can't use your speaker phone until they fix that. Next best solution is a Sense ROM with Sense Toolbox in Xposed and SoftKeyz, but then you can't use ART. But with Dalvik, I average 16-20 hours on a charge, and I charge when I sleep so... Not worried about ART.