What is the major theme in "The Open Window"?

For me, unlocking the theme of this short story
necessarily involves considering the character of Vera. Vera, of course, is the
storyteller without equal, who is quickly able to seize on details and weave convincing
tales to horrific effect. Note how she dominates the story - it begins with her words
and ends with them. We are told in the first sentence that she is "a very self-possessed
young lady of fifteen". It is clear that she sees in Framton Nuttel an object for one of
her stories, as she is quick to establish that he knows nobody from the area and thus
she is free to use her excellent wit and intelligence to create a fable that will shock
Framton Nuttel for her own amusement. She shows herself to be an excellent actor as well
as a storyteller. Consider how the author narrates her duping of Framton
Nuttel:

Here
the child's voice lost its self-possessed note and became falteringly human... She broke
off with a shudder.

She is
not only creative, but quick, intelligent and able to fool others into believing her
words. This is demonstrated yet again at the end of the tale when, nonchalantly, she
creates another tale to explain Framton Nuttel's swift escape from the house to trick
her family, telling the tale "calmly" with complete equanimity. Clearly this tale
celebrates the power that a good storyteller can have over a susceptible audience, with
Vera presented as the master storyteller who takes all victim to her powers of the
imagination.