phone format responsiveness

I’m using the London by Maelstrom theme (unfortunately no longer available, and I am unable to contact the developer) which is a responsive theme. However, when I view my website pages on a phone, the main page title text appear ‘cut-off’. I imagined a responsive theme would adjust the text size automatically according to the screen size of the device being used.

Please see website page :

I am new to WordPress and website developing so any help would be greatly appreciated!

Thank you!
Misako