Q:

How do i make React Native app look the same on iOS and Android?

I want to achieve the same look on both platforms, like Instagram does, for example. By default react native is bringing platform-specific UIs

A:

Yes, because that's the way it works, just like if you were using plain Java and Objective-C/Swift. React Native isn't meant to be a one size fits all approach, neither does it intent to be a framework where you can create an app that looks the same in all platforms. 
You'll have to customize the colors, views and images in each app, using the tools and UI frameworks and toolkits available in each one. 
Updated for more information
It's harder to answer that than you may think :), because the choose of the framework has direct impact on how you develop the applications and which technologies you and your team will have to use. For example, you may go with a hybrid approach (like Cordova) so you can use HTML and CSS, but that may be a shot in the foot depending on what you want to achieve. 
You could also go with Xamarin and its Xamarin.Forms UI framework (which I actually use and find it great), but then you'll have to stick with C# and still need to do platform tweaks. 
You may want tro try a (paid) product called "Native CSS" (https://nativecss.com, I have no relation to it) that allows you to use CSS in native code. 
The point is, without specific details of your needs, capabilities and even business decisions, it's very hard to securely say "go with this approach". 
However, In my experience it's not THAT hard to make the app look the same in Android and iOS, because all you have to do is to use each platform's tools to draw the UI the way you want. 

