Since Apple introduced dark mode, also known as negative polarity, in 2019, it has become increasingly popular. However, digital displays are typically optimized for office lighting, favoring positive polarity to reduce ambient illumination reflections, which aids in bright environments but reduces usability in dark ones. Most studies on the readability of negative polarity although focus on non-neurophysiological methods under normal illumination and in comparison to positive polarity, while mobile websites, used in dark illumination conditions, receive less attention in research. This study seeks to examine the effects of ambient illumination on the readability of negatively polarized mobile websites. We propose a research design consisting of an eye-tracking based, between-subjects laboratory experiment to examine these effects.