What is a theme of "Stopping by Woods on a Snowy Evening"?

At the most literal level,the central theme is undoubtedly the pleasure which a person derives from an awareness of his or her solitude or isolation.The speaker realizes that the pleasure he receives by appreciating the beauty of the snow-covered landscape is an act of stealth ,a guilty pleasure,if you will, and this realization intensifies his enjoyment.The speaker is therefore a man of heightened sensibility since he can enjoy the beauty of the enchanting snow covered landscape even though the property does not belong to him.It is a Romantic poem since it describes how Nature can evoke feelings of awe, enchantment through its manifold splendour.The poet realizes thathis natural surroundings are alluring and splendid yet he must continue his journey because as a human being his life is transitory and of brief duration.The phrase 'miles to go before l sleep' is therefore a reminder of the myriad mundane pursuits which a human being labours after before his demise overtakes him

One theme is the theme of isolation.  
This is probably my personal favorite theme of the poem. It's a lonely poem. The speaker is a man who has stopped with his horse beside a lonely wood. He doesn't know who the woods belongs to, but he is sure that the owner lives in the town that is in the area.

Whose woods these are I think I know.   
His house is in the village though;  

There's nobody else around. In fact, he is so alone that the only sound that he hears is the wind and the harness bells. 

He gives his harness bells a shake   
To ask if there is some mistake.   
The only other sound’s the sweep   
Of easy wind and downy flake.  

I never get the feeling that the man is scared by his isolation. In fact, I get the opposite feeling. I think that he enjoys his isolation and alone time. I feel that is especially evident by the positive way in which he talks about the dark and snowy woods.

The woods are lovely, dark and deep, 

The ending of the poem seems to indicate that he is tired and not looking forward to keeping those promises. Those promises are likely reminders of his busy, people-centered life. I always get the feeling that the man is thinking those final two lines with a heavy sigh.