Learn Spanish (Latin America): Si aceptas lo positivo, eres más feliz. - If you accept the positive, you are happier.

Lo is a neuter article. When you put lo in front of a masculine singular adjective like positivo, it turns the adjective into an abstract noun. Instead of referring to a specific masculine object, lo positivo means "the positive part", "positive things in general", or "that which is positive".

Why do we use eres (from ser) instead of estás (from estar) for "you are happier"? Isn't happiness an emotion?

While estar feliz is used for feeling happy in a specific moment (like being happy about a piece of news), ser feliz is used for being a happy person in general, or having a happier life. Because this sentence is giving general life advice about your overall well-being, eres is the best choice.

Does feliz change to a feminine form if a woman is speaking?

No, feliz stays exactly the same. Adjectives that end in a consonant (like the -z in feliz) or an -e usually do not have separate masculine and feminine forms. They only change when making them plural (becoming felices).

Why is there no accent mark on the word si?