When reading a cummings poem, it often seems that we "feel" the meaning of the poem rather than actively comprehending the words. cummings toys with this here in his use of language such as "syntax," "paragraph," and "parenthesis," academic terms which stand in contrast to the "feeling" which, the title states, is "first." The overriding theme of the poem, then, is that it can often be difficult to categorize our feelings—and that sometimes it is better not to try.

When we are overwhelmed with feelings, we do not feel compelled to "pay any attention...to the syntax of things," with the speaker telling his subject that a person preoccupied with doing so "will never wholly kiss you." To be too preoccupied with the "meaning" of life and the specifics of what is happening is to divorce oneself from the "feeling" of living. The visceral parts of the speaker understand what is happening: "my blood approves," while being a "fool" is acceptable when "Spring is in the world." ("Spring" may here mean a rhetorical spring, a sense of blooming and possibility.)

Ultimately, the theme of this poem is contained in the phrase "kisses are a better fate than wisdom." Life and love do not need to be constrained to a "paragraph," especially given that death will soon enough take us all—it is no "parenthesis" or unimportant piece of information to be bracketed away, but rather something that will put an end to feeling. As such, the "kisses" should be enjoyed while they can be.

The main theme behind this poem is logic versus emotions.  And, Cummings feels that emotions are much more powerful, joyous, and worth paying attention to.  He addresses the poem to a woman who he feels very connected to, and states that "my blood approves" of the relationship, and "kisses are a better fate than wisdom."  Even if his brain, and logic intercedes, and tells him all of the reasons that he should be cautious, or that they shouldn't be together, or the many, many reasons that they are unsuited or a bad match, he believes more in her "eyelids' flutter which says we are for each other."  He is telling her to stop worrying so much, stop thinking so much, and that "the best gesture of my brain is less than" the pure feeling and emotion of being with her.  He wants her to trust the feeling of happiness they have, to "laugh, leaning back in my arms" and to enjoy the time that they have together, and to leave all the fretting and worrying behind.  Life can't be calibrated and organized like "a paragraph", so they should just enjoy it in all its happy chaos.  I hope that those thoughts help you to understand the poem a bit better; Cummings can be difficult, as he throws all grammar, punctuation and conventions to the wind and writes in a very unusual style.  But, I hope this helped.  Good luck!