Best way for overwriting plugin css with custom one

I've to do very little changes to bbPress (forum plugin for wordpress) for a customer, at all about 5 lines css only. But I'm not sure where is the best place to put this css code. Modify the original plugin css files seems not to be a good idea because it may be overwritten on the next update and it's hard to see what was modificated.

My second idea was to place the css code in the main css-file of the theme. This seems a bit better, but the customer is using a paid-theme which get updates too. So these changes will be lost in case of theme-updates.

Where is the best place to put such css-modifications so that they're not getting overwritten by updates of wordpress, themes or plugins? Wordpress is developed very modular so it seems for me that this is not possible without extra-work (which has to be documentated and regarded on every update).