Is your site built using a well known CMS? If so there should be links to where you can buy new themes or templates If your site is bespoke then you can head over to sites like templatemonster and buy an html template and work that into your site.
and we were supposed to know that based on what information that you had given? What did you build your app with? the same advice follows - start with the suppliers of the app building software.
That's lovely, but if you want to "reskin" then you must have already created an app. Or are you wanting someone else's work that you can call your own? If so, I suspect it'll cost you lots more than US$50
You need to decide which app building system you want to use and then go through their forums and find out how it's done. This is an internet marketing forum and while some of the people here are programmers not many are likely to be able to give you information about the styling of android apps You also appear to find communicating in English difficult and may be better using a forum in your own language. We have lots of people here who have ESL and use google translate so that may be an option too. https://translate.google.com/