What is the theme of "The Lumber Room" by Saki?

One theme of "The Lumber Room" by Saki is the stultifying and unnatural conventions of Edwardian England's society vs. the natural world.

In Saki's story, the self-appointed aunt, who is representative of Edwardian society, is modeled after Munro's Aunt Augusta, who was once described by the author as

...a woman of ungovernable temper, of fierce likes and dislikes, imperious...and possessing no brains worth speaking of....

The Edwardian world of this aunt conflicts with the imaginative, free and unrestricted natural world represented by the creative Nicholas, who defies his aunt's limitations and dull conventional behavior. This theme is developed with Saki's inimitable use of irony:

When the aunt of conventional and irascible disposition fails to appreciate and enjoy Nicholas's boyish imagination in his act of placing a frog in his "wholesome" bread-and-milk for breakfast, she punishes him unjustly for what she perceives as impudence. Nicholas is prevented from accompanying his two cousins to Jagborough Cove, and he is forbidden to enter the gooseberry garden. But, the indomitable spirit and quick mind of Nicholas defeats these attempts to provide his cousins more enjoyment than he has. For, having discovered the key to the lumber room, a storage room where the most ornate and intriguing things are kept from the children, Nicholas secretly enters it and engages in flights of fancy. In this secret room, his creative imagination is greatly ignited by the scene woven into a tapestry that depicts a hunter and various animals. 

After a while, Nicholas hears the cries from his dictatorial aunt emanating from the gooseberry garden, but he cleverly turns her own orders against her, saying that he cannot enter because he has been forbidden to do so. Then, after she gives him permission, the mischievous and creative Nicholas objects,

"Your voice doesn't sound like aunt's,...you may be the Evil One tempting me to be disobedient. Aunt often tells me that the Evil One tempts me and that I always yield. This time I'm not going to yield."

Because he refuses to enter the garden and pull his aunt from a rain-water tank into which she has fallen, the disgruntled aunt remains there for an unbearable thirty-five minutes until a maid discovers her. After the other children return from a very disappointing afternoon at a beach covered by the tide and other discomforts such as boots that have been too tight, Nicholas delights in the ironic turn of events and his private victory of mind over convention.