Safari browser users
If you are a Mac user with a Safari browser you can still use developer tools to test, audit and benchmark your website, though the devtools and the type audits available in this browser are different. The first difference with Safari is you have to find, select and enable the devtools before you can start using them, though once enabled they stay accessible in your menu options.
Enabling Safari developer features
Start by opening the Safari menu and select Settings
Now select the Advanced tab (one with cogs icon) option
Then check the Show features for web developers tick box
Opening Safari developer tools
Now they are enabled you can quickly open the Web Inspector tool when needed by right clicking on the page, then select Inspect Element option.
Or use the keyboard shortcut command Option + Command + I.
Safari webpage audits
In the web inspector tool you have the ability to run some accessibility audits similar to Lighthouse, but unlike Lighthouse you need to select the individual or groups of audits you want to run against your webpage. Which in theory sounds great but their audit tool, limited audit options and the results they provide are more techy and seem designed more for actual web developers, rather than the general website owner. Unlike Lighthouse and PageSpeed insights with their more user friendly interfaces, which return a much clearer and easier to understand set of results, with error messages that have helpful guidance for fixing issues, plus suggested areas for improvement.
Darkmode in Safari
To toggle darkmode in Safari you need to have enabled DevTools and opened the web inspector. Once opened select the Elements tab, then from the toolbar just below elements tab click on the Override user preferences icon, now in the popover box under "Appearance" select the "Color scheme" option Dark from the pull down options.
Once enabled you will have to go through your website and manually check that all your webpage contents work in darkmode, as Safari doesn't seem to audit darkmode.
It is possible to test darkmode using PageSpeed, but to do this you would have to go into your CSS file and change the order of your base system colour from color-scheme: light dark; (light first) to color-scheme: dark light; (dark first). Though this will mean that your whole sites colour preference order will be changed for everyone, whilst you are testing and not just for you, so don't forget to change it back when you've finished.