What is the theme of the short story searching for summer by Joan Aiken?

People usually dont notice all of the good things in their lives until they are lost.

or...

We should protect what is most important to us. This is shown when the two main characters lead the greedy man, who only wants to make money off of the current most rare and valuable thing, away to a place far from the sun.

I also believe that one of the possible themes could be you cannot receive without giving. This is evident when Lily leaves her wedding dress at the cottage and Tom comforts her saying "You won't need it again." Lily and Tom received the gift of Ms. Hatching's hospitality and the gift of the sun in exchange for her wedding dress. This theme is expressed a lot more subtly and may perhaps differ upon your viewpoint.

Another theme is the destruction of the environment due to nuclear bombs. However, this can be tied into "We should protect what is important to us."