Can someone explain the genders in this sentence?
The word חלון is one of those masculine nouns with an irregular plural form that looks feminine. שבעת and פתוחים are masculine forms.
kol shiv'át ha-chalonót ptuchím.
Shouldn't this be either "all the seven windows are open" or "כל שבע חלונות פתוחים" to be a correct translation?
The number seven as you wrote it (שבע) is the feminine form. You need masculine for חלונות, which would ordinarily be שבעה, and then you need to make it into a construct for the definite form, resulting in שבעת החלונות.
They could have done whatever they wanted when they revived Hebrew and they did this.
what and make it easy? where would be the fun in that?
Isn’t this really translated as all the seven windows?
Yes, that's the literal meaning. But as far as I know, "the" is not necessarily in English.