Certain applications in Windows 8 display border of thick size. If you would like to reduce it without any hacks, try a simple and freeware app called TinyWindowsBorders.

The application helps you adjust the border thickness in Windows 8 with the help of sliders, anywhere between the ranges of 1-10 px.

TinyWindowsBorders has been designed specifically for Windows 8 users and is free for use. Moreover, the application is portable so it does not require any type of installation. Just download the application and start using it.
Download TinyWindowsBorders for Windows 8 from here.