Use calc() inside of clamp() to bypass SassC warning

What does this MR do?

MR !2960 (merged) seems to create an issue with the SassC compiler in GitLab upstream, see here

SassC::SyntaxError: Error: Incompatible units: 'vw' and 'rem'.
        on line 19993:29 of stdin
>>   font-size: clamp(1.75rem, -0.1388888889rem + 3.9351851852vw, 2.8125rem);
   ----------------------------^

The problem being that SassC does not fully support modern CSS3 functions like min(), max() & clamp(). Workarounds can be found here & here.

We decided to use calc inside of clamp. Once we upgrade to (dart) sass this workaround can be removed again.

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