Texstudio: How to change color of internal pdf viewer?

You can invert the colors. The setting is only accessible via the menu of the windowed viewer:

1. Open Viewer (embedded)
2. Switch to windowed mode using the toolbar.
3. Menu -> Configure -> Invert Colors.
4. Switch back to embedded mode using the toolbar.

The setting is persistent across viewer and program restart, so you have to do it onle once.

Strict inversion doesn't alter the contrast, so I prefer to change the page background while writing

\documentclass[letterpaper,12pt]{article}
\usepackage[usenames,dvipsnames,svgnames,table]{xcolor}
  \pagecolor{gray}
\usepackage[english]{babel}
  \usepackage{blindtext}

\begin{document}
  \section{BlindText}
  \blindtext

  \blindtext
\end{document}

I don't like the pure black background through the setting of Tim Hoffmann. Meanwhile I notice that we can apply the trick Tim Hoffmann proposed, "Options -> Internal PDF Viewer -> Paper Color", more flexibly. I just set paper color to light gray "#d1cfcf". Then I invert the colors to get deep gray background, which is the type I prefer.