Dora, welcome. i had no experience with less before beans. what i did was a little background on less just to have a basic understanding. i did proceed to check theough the 3rd party child themes as its a little different and it gives another understanding of working with beans.
to answer your questions yes you can use css. however less is quicker and less codes being used. i suggest you look through the documentation and this forum it is definitely educational and you'll learn some tips and tricks. hope this helps.