The colors of the theme are really nice but I would like this to also extend to the new tab page and other parts of the browser. For example, the built in dark theme, in Firefox, changes the new tab's background to a dark theme.

Edit: Thanks for the reply. You can add colors to the new tab page and sidebar by adding these lines to manifest.json file, unless you made your theme in a way that doesn't give you the manifest.json file. One of Mozilla's websites just lets you customize a theme and immediately upload it, but they also have another that lets you save and download it so you can continue editing the theme. The website is color.firefox.com and it lets you see the changes happen, to the browser, as you make the theme. This is just a piece of the code.
"ntp_background": "rgb(37, 0, 40)",
"ntp_text": "rgb(96, 195, 221)",
"sidebar": "rgb(8, 3, 41)",
"sidebar_text": "rgb(96, 195, 221)",
"sidebar_border": "rgb(96, 195, 221)",
"tab_line": "rgb(96, 195, 221)"
Note that the tab_line was already part of the theme but for some reason it was set to the wrong color, so I moved it and changed the set colors. My forked theme's colors are slightly different compared to your theme. Here is some extra things Mozilla lets you customize a theme. developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme