In terms of sonnets, if there is a "universal" theme (and this is a subjective question), love seems to be a central (and perhaps universal) theme.
Sonnets originated in Italy, perfected by the poet Petrarch. "Sonnet" comes from the Italian word "sonnetto," which in Italian means "little song." Sonnets were traditionally love poems. After the sonnet was brought to England, Shakespeare took it and made it his own. The English (or Elizabethan or Shakespearean) sonnet has fourteen lines, as does the Italian (or Petrarchan) sonnet. The rhyme schemes (pattern of rhyme taking place with the last word of each line) are different.
In looking at Sonnets 18, 65, 97, 106, and 116, each refers to the depth of the speaker's love, though sometimes more directly than others. In Sonnet 18, the speaker describes a beautiful woman: this was not unusual. "She Walks in Beauty" by Lord Byron was not a love poem at all, but a poem of praise for a beautiful relative. However, we can draw an inference about the speaker's love—he wants to immortalize her and protect her from death:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
In Sonnet 65 also speaks of beauty and the detriment of the passage of time to beauty—to all things—the reader must wait until the rhyming couplet (the last two lines) in order to hear the speaker share his feelings:
O, none, unless this miracle have might,
That in black ink my love may still shine bright.
Here, as in the first sonnet, poetry will preserve the essence of the speaker's love. In Sonnet 97, we must infer the speaker's love from the state of abject loneliness he experiences when the woman he is speaking to is away:
How like a winter hath my absence been
From thee, the pleasure of the fleeting year!
…And, thou away, the very birds are mute...
Not only is the speaker lonely for her, even the birds are silent!
In Sonnet 106, inference is also needed: for while the poet does not write about love, he does mention "worth." We can assume that it is not just her beauty that holds the speaker in thrall, but what she is: and the love he has for her. People can see how special she is, but cannot put into words how they feel—what they see.
They had not skill enough your worth to sing.
For we which now behold these present days,
Have eyes to wonder, but lack tongues to praise.
Finally, the speaker in Sonnet 116 does not hesitate to describe what love is:
Love is not love
Which alters when it alteration finds...
The speaker notes that love does not change when something new comes into the relationship. In fact, it remains as unchanged as the North Star is to sailors, who look to this cluster of stars to navigate safely home. Even storms in life will not shake true love.
O no! it is an ever-fixed mark
That looks on tempests and is never shaken;
It is the star to every wandering bark,
Love does not change as time ravages the body over time; beauty may fade, but love never will. Love is not a brief event—it is "forever"—lasting "...even to the edge of doom. " The speaker says that if anyone can prove he is wrong, he has never experienced love.
Shakespeare wrote 156 sonnets—a favored topic is love. He may speak of beauty or the passing of time, but his primary focus is on love. If there can be a universal theme, it would be love.
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.