What you are looking for is probably CSS / HTML classes. See the documentation for more information. You were actually quite close to the “good” solution (the only difference is that id=marking only works for a single tag, that is, you are not supposed to have several tags with id=marking — hence the name id — whereas with class=marking, you don’t have this restriction anymore)!
Also, you may be interested in css variables to set something like emphasis=red and, in the rest of your css declaration file, you can use emaphsis instead of red, so that if someday you want to change your emphasis color to, say, green, you only have to change emphasis=green instead of changing it everywhere.