One possible theme could be grief. As she is a widow, she must be dealing with the loss of her husband. Another theme might be loneliness, as she may feel isolated without her partner. And also, there could be a theme of new beginnings, as she has to start a new chapter in her life.
Well, in 'the good widow a novel', likely themes could include love in different forms. She may have memories of the love she had with her late husband, and perhaps she'll experience new forms of love from friends or new relationships. Also, the theme of identity is possible. She was part of a couple, and now she has to redefine who she is as an individual. And of course, the theme of change, as her life has been drastically changed by her husband's death.