High contrast mode
While a good default goes a long way, there will always be people for whom it will not be enough.
This includes people with permanent vision impairment, but also includes temporary and situational impairment such as playing in a sunny room. Others still have more specific contrast requirements, such as people with cataracts who need dark backgrounds due to diffraction/glare encountered from bright lights drowning out interface elements.
Ideally provide a selection or configurable colours, but at a minimum offer a single mode with very high contrast, achieved through design rather than a filter. This functionality is particularly powerful when combined the option to increase text size.
Choice of preset high contrast modes in Windows, with the option to configure each individual color
Next: Text size