I reckon they mean something conventional, but are there any other meanings more than that? What are other usages of them? The author of the book, published in 1982,is a British.

I would also like to add that although "chocolate box" is an established phrase in the UK, it is not a common expression in other English speaking countries like the US. Hope this helps!

As far as I can tell, the meaning is that although Impressionist art was originally groundbreaking (and most people did not like it), it is now so widely accepted that it is often used for the images on calendars and chocolate boxes. (I've never seen Impressionist art on a chocolate box, but maybe this used to be common. I've certainly seen many calendars that use Impressionist art.) In other words, this style of art has become so commonplace that it is now used on everyday items like calendars, even though at first, it was so strange and original that people didn't even accept it as art.

They just mean you see impressionist art everywhere, including on the cover of calendars and boxes of chocolates that are sold in stores. So impressionist art has mainstream acceptance nowadays.