Since the Dark Theme in Firefox 68.0.2 still has a flashing white page when opening a new tab, it’s no useable dark theme in my opinion.
That’s why I prefer ShadowFox (Github) instead of the native dark theme. Easy to install and many ways to do further customizing.
Firefox Add-ons I use:
Also I like bookmarks in the toolbar centralized. Just append the following lines to the userChrome.css.
The file is placed in your Firefox Profile: ~/.mozilla/firefox/profile_name/chrome/userChrome.css
#PersonalToolbar {
display: flex !important;
justify-content: center !important;
}