Better Brightness Slider expands the default brightness slider, making it range from -100% to 1200% (fullbright) instead of 0% to 100%. It's a clientside mod, so it isn't required on servers.
The feature is ported from Boosted Brightness, if you want a fabric version, download it there.
The mod should be compatible with Optifine and Rubidium / Magnesium (Forge Sodium).