Theatre dimming of interior lights

I would like to add theatre dimming to the interior lights of my 1992
C900 convertible (together with courtesy lights in the front foot
wells - can't think why there aren't any!).

At present the lights are shut off by a time delay relay, made I
expect by Bosch. It would be very easy to pull this and replace it
with a similar relay that provides for theatre dimming at the end of
the time delay. Does anyone know of any such relay that could be used?

Alternatively, is there another way of doing this, with perhaps some
"add-on" electronic component? I've been unable to find out how this
dimming works, other than by an electronic "black box", even after
exhaustive searches on Google.

I'm very interested to hear how anyone might have done this
effectively!

Thanks in advance.

Of course, two minutes after posting the above, I tried another search
and hit paydirt for electronic circuits for theatre light dimming,
using the search term "dome light dimming". Silly me, looking for
theatre dimming!

However, if there is a simple relay swap, I'd really like to hear
about it, rather than build a whole new circuit and incorporate that
with the existing relay, or in place of it......

TIA