@dragonsneagu95 I did the following on a 4.5.7 version of Core, to get a dark background on the Services page.

In /usr/local/nagios/share/stylesheets/common.css, I added the background-color line and the td.infoBox rule.

body {
  margin: 0.8em 0.8em 2em 0.8em;
  font-family: Geist;
  font-weight: normal;
  font-size: 10pt;
  background-color: var(--background);    /* This makes the background black */
}

/* This overrides the color of the text in the top left hand corner */
td.infoBox {
  color: var(--foreground);
}

In /usr/local/nagios/share/stylesheets/status.css, I added this rule, after the first th.status rule, in my case that was line 14.

th.status {
  color: var(--foreground);
}

After updating, you should be able to do shift - reload with your browser to have it pull in the changes.

There may be other tweaks required, but hopefully this will get you started.

@dragosneagu95, You verified that the files you copied over the stylesheets folder into /usr/local/nagios/share? Also note that you may need to clear your browser's cache for your Nagios server for changes to reflect properly.