NOTICE: I unfortunately lost the signing key and won't be able to update this app further. I will be developing a new version with all the suggestions I've received.
As my search for offline dice rollers wasn't really successful - or at least not to the extend Hey guys, so today I bought DnD 5e off Amazon, my first rpg. EDIT: edited the download link, you should now be able to download it.
Free Download Dice Roller - Easy-to-use and handy dice rolling but less spiffy- looking solution such as the d20 Dice Bag over at Pen, Paper.