Change 404 Title

Hi,

I successfully changed the template of the 404 page, but I searched how to change the title of the page.
It seems the title appears to be handled by the theme, but I didn’t find where.

For instance, I changed the template PHP code and now the website shows what I added, but the title of the page can’t be changed with the title HTML element… it still shows “Page not found-” in French.

Can someone say to me how I can change it ?

Many thanks,
Logan