NORTHAMPTON — For decades, Thornes Marketplace has offered shelter to shoppers within and to pedestrians without. On Monday, the collection of stores will begin putting new face to its exterior. The iconic green awning on Main Street, a Thornes fixture since the 1980s, is in disrepair and needs to be replaced, the owners said Friday in a press release. While they’re at it, Thornes will install a new sign that harks back to the one on McCallum’s Department Store, which occupied the site for more than a century until it was sold and became the multi-shop indoor mall that is Thornes.