Wow! Very nice as is. I wouldn't worry about the lining as it doesn't show. I like that it looks understated with clean lines and dressy enough for work or other occasions without looking like it's part of a costume. Good job!
I also second other people's suggestions. In addition, if the shed's roof can support the weight, I think the entire structure would be lovely if it had a living green roof with plants that need minimal care and nutrients.