Pseudo-transparency of lualine and tmux statusline

[Gigibeau]

Hey there,
I took some influences by this config especially in terms of design and everything works as intended. The only think I don't seem to grasp is how the transparency of lualine and statusline works. Is this something that actually works or is it something that happens in post production when putting together the video?

My terminal looks like this but a bit later in the video the bars turn transparent.

I'm using kitty instead of iTerm2 and I'm also on macOS.