the best place to change CSS settings depends in your circumstances
The chief difficulty of making custom changes to parts of standard Moodle, or QuizPort, or your nice theme, is that when you want to get a new version of Moodle, QuizPort or your nice theme, then you have to copy across your customizations. That can quickly get difficult.
Therefore, and I recommend this especially if you are considering making a number of CSS changes, I suggest you make a new theme, based on your current theme. I believe in the theme settings you can say that your new theme is based on another theme, so as a rule it will use the settings from the other theme. Then, in the new theme you can put just the settings that are different.
I don't know a whole lot about making new themes, but I feel I know enough to be able to make the above suggestion. For more info, please consult the Moodle docs or the Theme makers forum.
all the best
Gordon