If you have the exact solution, please pass on the modification as well and the exact path of the CSS file so that it could be helpful for other developers.

Once you post your code, then your answer should be marked as a solution.

Most simple way is edit pos index file under the technical/views between <style> body { background: #222; } </style> for example
<style> body { background: #222; }
.pos .numpad button { height: 108px !important; width: 108px !important; }
</style>