Some beers are hard to stylise, misusing the word stylise. There are grey area between styles, and disputes over style descriptions. Mild originally meant young, not aged. Nowadays milds tend to be low abv, low bitterness, low hop aroma. Around where I live Hydes and Holts pubs both sell dark milds on keg taps, cold. The difference between these and Guinness is not huge, imo. They are weaker and less bitter, and less dry, but you may struggle to decipher them if blindfolded, maybe. A dark mild will be less roasty than a stout, but not so much that it would be obvious, I suspect.