If it's Helvetica, you don't need to import the font first before using it. I mean Helvetica is actually the font fallback Fandom used in case the default Rubik font doesn't work. Now, do you want the font to be used only on the green area, on yellow, or on the red area?
If it's the green area, here is the code you should use:
.page__main { font-family: helvetica,arial,sans-serif; }
If it's yellow, use this:
.page { font-family: helvetica,arial,sans-serif; }
And if it's red, you should use this instead:
.main-container { font-family: helvetica,arial,sans-serif; }
I don't recommend you to use the last one though, as it is likely a Customization Policy violation.
Also, I set Arial and generic family Sans-Serif as fallbacks if Helvetica doesn't work; however, feel free to change it to other fonts as long as it isn't a font requiring you to import it first. If it is, you must import the font first, the step of which is available on the link Muchos shared