you can use double quotes as well
"I really don't care." He thought to himself as she talked about her summer vacation.
It's like your mind is speaking, so that's ok. Lets people know to read the line in the character's voice. You just have to make sure that you write "they thought" or something along the lines of thinking afterward so we know it isn't speaking.
Another thing you can do is italics, which was brought up before in the thread.
I really don't care. He thought to himself as she talked about her summer vacation.
And tbh I see italics most often when it comes to how you write thoughts.