Are you for real? You can change the brightness in the settings area, and you can also put a brightness toggle in the action centre so you can change it directly from there.
Why didn't you say so in the first place? If/when you have cyan you can got to the display settings to get more control over the brightness using a slider.
Brightness slider is available for new phones launched with 8.1 OS and Cyan firmware for eg: 630. Secondly, there is no brightness slider app that helps you to achieve your goal. This is where WP distincts itself from Android by not giving blind control over OS to third party apps and I personally like it