[X Style (Pure) Themes and Apps] Mejilan:
Hiya. I wound up playing around with this for around two hours before I gave up, uninstalled it, and went back to Layers Manager and Coalesce. I have a number of issues with the app in its current incarnation, which I will begin listing below in no particular order:

1) You really need someone to give it a more professional English language pass. I'm not trying to be offensive, but I'm assuming that English is not your native language. There's nothing wrong with that, but you definitely should have someone more comfortable with English rewrite the app. Especially since you're charging the equivalent of USD $5.22 for the Pro version.

2) During the introductory (first-run) splash screens, some background/font color combinations rendered the text completely unreadable. I want to say that it was the fourth or fifth screen... It had a white font on a bright yellow background. It was painful.

3) The first time patching caused a silent reboot, which I suppose is fine (though perhaps a little surprising). But what I didn't expect is that some default layers would be installed and activated upon reboot. Some of those defaults were just... garish. I think IF you absolutely have to force the installation of default layers when patching, then those layers should be as close to stock as possible, so as to not scare new users into immediately uninstalling the app. If you'd still like to have some kind of obvious before/after contrast, then perhaps the defaults should hold to Google's recommended Dark Material Design language. I, however, would recommend NOT forcing default layers on a user and just let them play with whatever they're comfortable with. That, or include one or more default reskins that they can apply at their own discretion.

4) Not a fan of the replacement nav bar icons. Not a fan at all. Maybe include a toggle to keep the ROM's stock icons? Bonus points if you can theme the accent color of the stock icons.

5) Limiting the trial to only 15 layer applications is perhaps a bit stingy. Especially for first-time users that have little to no experience in theming (such as me). Another idea that you might want to consider is to NOT count a re-application of a layer against the 15 total if it's the same as the last application performed in-app. For example, let's say I'm playing around with theming Settings, confirm, apply, and decide that I just don't like the final result. If I go back into theming Settings, make a couple of changes, and re-apply, it probably shouldn't count as a second hit. Only count it if the app is reopened or if I've moved on to a different category. This would allow for some tweaking without "breaking the bank," so to speak. Especially when users are still learning how the app works. Another idea is to just allow a 30 to 60 minute grace period after opening the app for the first time where theme applications don't count against the total. Not sure how feasible this is, or if uninstalling and reinstalling the app would reset that.

6) I had some pretty serious issues getting colors to "stick" or apply. I figured that for my first stab at theming with OvO, I would attempt to recreate some aspects of that lovely Coalesce Layers Theme available on Google Play. I had screen shots up on my PC and a color picker ready to grab RGB and Hex values of colors I wanted. And I focused primarily on theming Settings and SystemUI. SystemUI theming seemed to work without any major issues. I got the primary, secondary, and accent colors more or less to where I wanted them. But when attempting to theme Settings, I could not get any of the colors to stick. I tried entering the Hex values, hitting Pick or Select (as applicable), and then hitting Apply, but the colors remained as they were in their defaults. I also tried using the RGB sliders to input RGB values of my preferred colors instead. Same issue. I tried copying the color codes used in SystemUI (which did work) to Settings, and that failed to stick as well. I tried rebooting, thinking that would fix the problem. Nope. The colors just would not stick. I actually wound up fiddling with this enough to exhaust all 15 of the free trial-period applications.

7) So with that last point in mind, I figured I'd see how much the full version of the app cost. For a couple of bucks, I figured I'd stick with it and try to power through my issues. But when the prompt asked me for USD $5.22, I cancelled, uninstalled the layers, and eventually uninstalled the app.

I think the concept behind OvO is a fine one, but feel that the execution leaves a lot to be desired. The way the trial is set up punishes first-time users. It's neither user-friendly, nor accommodating to first time users. The default layers applied upon patching are in no way aesthetically neutral. Some of the combinations are just painful to behold. Hangouts was a garish mishmash of black, green, and yellow, if I recall correctly. Due to a shifting background color, sometimes the app's very text is completely unreadable. The text that is legible is poorly written, riddled with grammatical errors, and occasionally reads like border-line nonsense. OvO is badly in need of polish, especially at the price you're asking for to upgrade to a the Pro version.

I wish you the best of luck, however.

Edit - Moto Maker XT 1575 activated through AT&T, running Stock 6.0 MM, TWRP 3.0.0-1, SuperSU 2.67 Pro, Xposed v80. No force closes or crashes or anything of the sort.