The Theme object encapsulates all elements of a single weblog theme. It
is used mostly to contain all the templates for a theme, but does contain
other theme related attributes such as name, last modifed date, etc.
Lookup the specified template by link.
Returns null if the template cannot be found.
NOTE: for themes we enforce the rule that
Theme.link == Theme.name
So this lookup is basically the same as lookup by name.