How change the theme of abrowser (or another free web browser) so be darker?

Are you referring to the theme of the tabs and menus, or of the web
pages themselves?

By default, the tabs and menus should just inherit your GTK3 theme.
Assuming that you are using MATE, you might only be modifying your GTK2
theme if you are using the "Appearance" menu under mate-control-center.
You can set the GTK3 theme using gnome-tweak-tool or by editing
~/.config/gtk-3.0/settings.ini (see here[1])

If you want Abrowser tabs and menus to have a different theme from the
rest of your desktop, or if you want to actually change the look of web
pages, then I think you'll need to install themes and/or addons.

Refreshingly, Mozilla requires addon developers to specify a license
when uploading their addon to addons.mozilla.org, so it is very easy to
check whether such addons are free. For example, this addon[2], which
allows you to modify the look of individual web pages, is GPLv3.

Unfortunately, it looks like they don't require users to specify a
license when they upload a theme, so if you care about free culture you
might need to dig for the license of a theme that you use.

[1] https://wiki.archlinux.org/index.php/GTK+#Basic_theme_configuration
[2] https://addons.mozilla.org/en-US/firefox/addon/stylish/

I use the addon "Dark Background and Light Text". It is a life changer and it is free software: https://raw.githubusercontent.com/m-khvoinitsky/dark-background-light-text-extension/master/LICENSE

The colors I use are in the image below.

I don't recommend add-ons that invert colors as these don't work well on some websites (also these add-ons might become unusable in future releases if they become unmaintained and they probably are not compatible with all Mozilla based browsers such as Seamonkey or Pale Moon based ones).

My advise is to find a dark (but not too dark as black background is not fully compatible with some websites) system GTK theme (I'm using Arc-Dark) and then to select in your browser's color preferences (preferences->content/fonts and colors->colors) "use system colors". You can customize some additional settings such as the color of text/visited/unvisited links.

I'm using a dark grey background so that black text (if presented by the website such as this one for icons such as "Attach") is also visible. Of course this means that once selected all applications will use the dark system theme so if you want dark text in the browser and prefer a light theme for other applications then this method is not ideal and the add-on method or dark theme setting in the browser (if available) is preferred.

If like me you prefer a uniform look across all applications then you will be glad to know that GTK themes can be used in Qt applications as well (after editing some text files), so Qt applications without a dark mode can be made to look almost the same as the dark system GTK theme.