Latex document background looks grey

If you don't want the gray background, you can disable dark mode, or use \pagecolor{white} (which requires that the xcolor package be loaded).

Code:

\documentclass{article}

\usepackage{xcolor}
\pagecolor{white}

\begin{document}
    Hello World!
\end{document}

Another option that does not require changing the actual LaTeX documents appears to be adjusting the TeXShop theme. If you head to the settings and adjust the color highlighted in the screenshot from grey to white, you should be able to get the white background as desired.