Question

For example, if I write:

background-color: black;
color: white;

or

color: white;
background-color: black;

Is there any difference between the two and should I care about order?

Answer 1

No. You can write them in any order. In the case of duplicates within the same selector, the last rule wins.

p {
  color:blue;
  color:red;
  color:green; /* Green wins */
}

When dealing with multiple selectors, rules that come later will override earlier rules, unless the earlier selector is more specific.

For example, this code will turn all your paragraphs green:

p { color:red; }
/* ... */
p { color:green; }

While this code will turn all your paragraphs red, because the first selector is more specific.

body p { color:red; }
/* ... */
p { color:green; }

While the order of rules within a selector may not matter, I would take care to write them in a predictable order (position rules before sizing rules before colouring rules, for example) simply to make your CSS consistent and slightly more pleasant to maintain.