Learn Portuguese (Portugal): Ela sente frio à noite.

In this sentence, frio is a masculine noun meaning the cold or coldness, not an adjective. Because it is a noun, it has its own fixed gender and does not change to match the feminine subject Ela.

I usually see someone being cold translated using the verb ter. Can I use that instead of sente?

Yes! Ela tem frio (literally she has cold) is actually the most common way to say she is cold in Portuguese. Ela sente frio (she feels cold) is also perfectly natural and means the same thing, just slightly emphasizing the physical sensation.

Why does the word à have an accent pointing to the left?

That grave accent shows a contraction. It combines the preposition a (at or to) with the feminine definite article a (the) that belongs to the feminine noun noite. In Portuguese, we literally say at the night (à noite).

I sometimes confuse the verbs for to feel and to sit. How can I tell which one this is?

This is sente, the third-person singular form of sentir (to feel). The verb sentar (to sit) is a regular -ar verb, so its third-person singular form is senta. A good trick is to remember that sitting is usually reflexive, so for sitting you would usually see ela senta-se.