Can you state a theme for the poem "Because I Could Not Stop For Death...?"

Alright!  Great!  Another poem question...I love these!  In order to understand the theme of the poem we have to look at what this point is about.  I am using a particular translation of the poem but it should be fairly similar to yours:

"Because I could not stop for Death,
He kindly stopped for me;
The carriage held but just ourselves
And Immortality."

Life just sort of keeps going...nobody plans to "stop for death."  This is a bit of dark humor.  Death will come for you when it is your time, ready or not.  Apparently, you get some personal time with Death to boot.

"We slowly drove, he knew no haste, 
And I had put away
My labor, and my leisure too,
For his civility."

Death feels no need to move quickly once it has you.  In death there is no need for work or fun activities.  Those things are done and in the past.

We passed the school, where children strove
At recess, in the ring;
We passed the fields of gazing grain,
We passed the setting sun.

Children are a good symbol for the future, that is, they are young and have a long future ahead of them.  Fields of "gazing grain," perhaps representing middle age, and the sunset, symbolizing the end of one's life.

Or rather, he passed us;
The dews grew quivering and chill,
For only gossamer my gown,
My tippet only tulle.

The sun moves past them.  It's getting chilly.  She is wearing a gown made of cobwebs and a scarf that was made of netting.

We paused before a house that seemed
A swelling of the ground;
The roof was scarcely visible,
The cornice but a mound.

It's not really a house, it's a tomb.  That's why it is so close to the ground. She's headed for the grave.

Since then 'tis centuries, and yet each
Feels shorter than the day
I first surmised the horses' heads
Were toward eternity.

Death really isn't so bad, at least it doesn't seem.  Centuries have passed but each feels like less than a day.

The theme seems to be that Death is not something to be feared but is a natural part of life.  The character in the poem does not fight or run from death.  She does not attempt to stop the carriage nor does she react to seeing the schoolchildren.  Her tomb (she must have known what it was) looked like a home to her.  Death, indeed, treats her very civilly and is not something to be afraid of.