Some themes don't change NavBar

So i bought Xperia M5 a month ago and i love the phone and it has a grate thing ability to change themes i was always crazy about themes.Now there is a lot of themes online and on Google play especially on XperiaBlog anf with these themes you can change NavBar/Soft buttons/Navigation button.But in my case i can't at least not with all,themes that are made by Sony works fone well maybe two or three don't but others from online/XperiaBlog don't work,the wallpaper and all other stuffs changes except NavBar,So is anybody else having this issue or just me?
Trying to do this on Xperia M5 E5603 from 2016,lollipop 5.1.1 30.1.A.1.33 (that's the latest update for my CDA/SI code)Thanks

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