Make best use of new Org 9.5 faces

We need to refine the new faces we implemented in Org 9.5 with Gustavo Barros (@gusbrs). This is a continuation of issue 208, which brought us the modus-themes-org-agenda customisation option.

Tasks to be done:

• Implement the org-agenda-structure-filter, org-agenda-structure-secondary faces. How should they look?
• Configure the org-imminent-deadline face. We do not want it to be bold, right? What happens with org-warning? I say we keep that as-is for older version of Emacs/Org which use the themes.
• Can we make use of org-agenda-date-weekend-today without affecting the modus-themes-org-agenda? In other words, can we make it do-the-right-thing for current and older versions of Emacs+Org?
• Check whether we should decouple the styles of org-agenda-calendar-event, org-agenda-calendar-sexp, org-agenda-diary and how best to do it without complicating modus-themes-org-agenda.

Anything else?