User interfaces serve as an intermediary between users and computer systems. Over the years, they have gone through various changes and trends. One such trend that emerged at the end of the last decade is dark mode. The goal of this paper is to study the influence of various background colors on text legibility. By studying the impact of dark backgrounds on reading time and accuracy, an attempt is made to find the optimal background color for dark mode. The first part of this thesis discusses the characteristics of user interfaces, and the history and characteristics of the dark mode. The second part of this thesis is research in which 46 respondents participated in reading texts on different colored backgrounds on a mobile device. The results show that the choice of background color has a significant effect on reading ability. A specific shade of dark gray (#121212) was shown to significantly reduce the number of reading errors, while dark blue (#15202B) was shown to be the fastest to read.