Is it appropriate to bring coffee and cookies to a PhD defense?

How I have seen it handle in France: usually, no food and drinks is given out during the defense, except for a bottle of water and a glass who are set on table of each member of the defense committee. However, as you say, it may be cold outside and committee members may be glad for a hot drink and/or maybe something to eat (depending what time it is). So, it is quite customary for the thesis advisor(s) to actually invite the committee members to join him some time before the defense starts, where they can make small talk and drink coffee. If the defense starts early afternoon, they committee can even go to lunch together, giving them an opportunity to discuss practical stuff (who will chair the committee? what order will people talk in? etc.).

Then, after the defense is concluded, there is a celebratory “cocktail” (buffet and drinks) to which the defense committee is invited, and to which they usually attend at least for half an hour.

In mine, there are no conventions per se, but I've seen people get food ready for just after the defense. Since it's extremely rare to get to a defense and fail it, I'm not too concerned about the 'foregone conclusion' problem.

As others pointed out, the answer depends on the country, particular university, as well as local habits at the department.

Just to complete the international picture:

In Germany (at places I had an opportunity to observe), it was customary to provide a treat in a style of "wine & cheese" of course modified to local tastes (meat in Germany :-) ) including a glass of sparkling/normal wine (usually "Sekt"), but all that after the defense. People would normally order the food by a local butcher (many of which specialise in this kind of stuff as a side business).

The same would go for some Eastern European countries, like Czech Republic, or Slovakia.

In the Netherlands, at places I saw, there is often a small reception after the defense too.