Adding horizontal scrollbar

Hello

I'm using this nice theme to build our site, but I'm having some problems with the resize and scroll bars behaviour. I want to add an horizontal scrollbar when the size of the screen is small instead of having the menu in two "rows" (see screencapture attached).

Is this possible ¿?

Thank you very much and sorry if this question is quite obvious but I don't know how to solve it...

Thank you,

KiM