The main theme of this sonnet, like so many of Shakespeare's sonnets, is love. In the poem, he is talking about the constancy and permanency of love.
In this sonnet, Shakespeare talks about how love does not change. He says love does not change depending on the circumstances. He says it does not change over time. He says it stays solid like a lighthouse.
He says that he is absolutely sure of these things -- if he is wrong, there has never been love in the world.
There really is not any other theme going on in this poem.