I've generally thought that เขา is the common way to refer to both males and females; and เธอ is a somewhat cutesy way to refer to females.

But I've recently been instructed that เธอ is the common way to refer to females, with เขา mainly used in formal situations. Is this the case? For example, in everyday conversation, when somebody is referring to a female, they would use เธอ the majority of the time?

Thanks.

เขา can mean, male female and them.

I tend to use เขา in formal situations and เธอ in less formal situations where the people I am talking to know to whom we are refering.

If the person is unknown I use the formal เขา.

เขา is general purpose, neutral in register.

เธอ is usually used to refer to someone you are very close too, often a girlfriend or (less frequently) boyfriend. Almost every Thai pop song appears to have a line ฉันรักเธอ. Generally, เธอ is best avoided unless you're very confident in its use. เขา is always a correct, safe option.

The way I learned it is that เขา (or เค้า) is the normal, polite 3rd person pronoun (he, she, they)

เธอ as a 2nd person pronoun (you) is used with people you are very close with, and besides that you'll often hear it in movies and songs

เธอ (as well as หล่อน) as a 3rd person pronoun especially for females is outdated and usually only found in written language, I wouldn't use it in a conversation.

I had it explained to me that in spoken Thai เชา (s/he) refers to male or female (also in the conversation part of stories and novels).

But in formal or written Thai, เขา refers to males only and เธอ/หล่อน for females.

In spoken Thai หล่อน (you) is mostly used by กะเทย when speaking to girls.