Adding adverts Q

Hi there, I’m not terribly experienced with WordPress and still learning basics. I have Flexibility theme for my website which works well. However, I want to add adverts to my site – I don’t mean Google ads – but the set of boxes that are often seen. I plan to approach specific companies so need to be able to dictate the ads. I’m suprised that Flexibility doesn’t seem to have any option to have the ad boxes and I was wondering if this is something I can download as a plug-in and encorporate, or does it mean I need to change my theme to get one that has ad boxes?
Thank you
Sophie
https://tajiktourism.com/