Apply expl3 style

[Andreas Matthias]

How do I apply the "expl3" style to the current buffer?

I understand that the "expl3" style is applied automatically if the buffer
contains e.g. "\RequirePackage{expl3}". But how do I apply the style
if \RequirePackage or \usepackage is missing?

I tried to load the style manually with

  (TeX-load-style "expl3")

but this doesn't seem to apply the style.

Best regards
Andreas