女性 means “woman”.
は is the particle which marks the topic of the sentence.
誰だって means “no matter who” or “anyone”. For example, 誰だってできる meaning “Anyone can do it”. You can also use other WH-words such as, 何だってできる meaning “I can do anything”, どこだって行ける meaning “I can go anywhere”, いつだっていい meaning “anytime is fine”.
美しく means “beautiful” and なれる means “can become”.
When you want to say “to become an i-adjective, in this case 美しい”, just replace the い with く and use the verb なる which will be 美しくなる meaning “to become beautiful”. If it’s a na-adjective such as 元気, simply add に and use なる which will be 元気になる meaning “to become well”.
As なれる is the potential form of なる, 美しくなれる means “can become beautiful”.
The の particle is used when the speaker wants to emphasise the statement to convince the listener, usually used with the declarative auxiliary verb だ.
But in this sentence さ is used instead of だ in order to make the speech sound cool, dry, frank and more casual. It’s often used by optimistic and snobbish men like him who is a typical rich boy from an upper-class family. Although, in this usage, you don’t usually hear it in real life as it sounds cheesy.
The other さ particle which is often used in real life is the one Maruko used; そりゃ花輪くんのお母さんは若くて美人だからなんでも似合うだろうけどさ meaning “Hanawa-kun’s mom is young and pretty so of course everything would look good on her”. This さ can be used multiple times in a sentence to adjust the tone and in this usage both males and females can use it. It can also indicate that there is something the speaker wants to say after. So Maruko wanted to say “Everything would look good on your mom but not on my mom”.