Hi there,
You’d be able to do this with CSS.
Making the background black is simple enough:
body {
background-color: black;
}
But to make the text white you’d need to find every piece of CSS in the theme that controls the font color and write new CSS to override it. This means manually checking every single piece of text appearing on the site and finding the right CSS selector to target that piece of text so it can be overridden. That makes this a complex change, and is beyond what we can help with here.
You might also be able to do this using a plugin. I found some suggestions in a tutorial from WP Engine, here: https://wpengine.com/resources/how-to-customize-a-wordpress-themes-colors/
