What details show that the mood inside the house has changed in "The Monkey's Paw"?
I suppose you are refering to the way in which the house of the Whites changes after their sons tragic and unfortunate death, just as Mr. and Mrs. White themselves change and develop through this tragedy in their lives. This is shown most clearly at the beginning of the third section of this classic horror story, when Mr. and Mrs. White have buried their only son and have to try and return to their former lives. Note how the house is described:
In the huge new cemetery, some two miles distant, the old people buried their dead, and came back to a house steeped in shadow and silence.
The house, symbolically, is "steeped in shadow and silence" precisely because the lives of Mr. and Mrs. White are themselves "steeped in shadow and silence" as they are overcast by the death of their son. The passage goes on to describe their empty, seemingly meaningless lives as they try and absorb the brutal reality of their son's death and the emptiness of their lives without him. Whereas before at the beginning of the story the house, even in the midst of inclement weather, was full of light and warmth, the house now is characterised by darkness and gloom, aptly mirroring the feelings of its owners.