im using Linux (Ubuntu 16.04) and all games i have tried in Linux that require mouse aim the mouse sensitivity is either far too fast or far too slow, so can we get a mouse sensitivity slider in the itch settings so we can adjust it in game and not have to tab out and mess with the system settings to get the game setup correctly?

adding the setting to the app would not let you adjust it in-game, you'd still have to tab out and change it in the app. Personally I think a setting like this is best left your desktop environment to set. I think it's probably more worth it to request developers to add setting for configuring the mouse sensitivity, since ideally you don't want to change the system mouse settings just to play one game.