Can a wall switch be used in a DC circuit?

Sure.

It probably won't light up.

My limited experience of switches of this type is that two pieces of metal (contacts) either touch or they don't. During the transition between on and off you may, in some circumstances, get and arc. In extreme circumstances you may burn out your contacts or weld them together. But none of that seems likely if you're switching a 12V 300 mA fan.

Then it probably won't light. A schematic showing the type of light and how it's connected would make it easier to answer that question.

It all depends on the ammount of current and if the load is inductive like a motor.

Many times you have to derate the switches current by a factor of 5 to

So using a standard AC wall swith on DC you would be plenty safe switching a 12 volt DC motor that draws less than one amp.

Can I use the dimmer if there is one present?

Few if any 'dimmer' devices that are used with AC are also workable with DC circuits. A few LED devices use a 0-10V protocol for dimming, and those controls have some applicability.

A 120VAC dimmer will not function on 12VDC.

RL

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Notice that the same relay rated for 250VAC and 10 amps is also rated for 30VDC and 10 amps. Same goes for "thermal - magnetic" circuit breakers intended for domestic AC use.

..... Phil