Lots of positive points here, especially on things that are commonly overlooked (security headers, SSL configuration, compression, etc.), so congrats!

Some quick comments:

1) Under the tools listing on the home page, I'd make the entire block of each tool (name, icon, description) the clickable link, not just the words "Use Tool"

2) When in dark mode (which was auto-sensed, good job), every time I choose a new page it starts in light mode (flash-bang!) and then fades into dark mode.