Vertical alignment of toggle inner circle is off for some text sizes

[sbates-idrc]

Th inner circle in our toggles is only centred vertically for some text sizes (see screenshots).

100%:

110%:

125%:

There is maybe some non-relative unit specified that causes the inner circle's alignment not to work for all text sizes.

Original Jira issue: https://fluidproject.atlassian.net/browse/C2LC-400