take first theme in list if chosen theme is not in list
fixes #15 Bfore applying the theme, it checks if it actually exists in the list of available themes. If not, it takes the first theme available (in our case, by alphabetical order, Acario).
fixes #15 Bfore applying the theme, it checks if it actually exists in the list of available themes. If not, it takes the first theme available (in our case, by alphabetical order, Acario).