Could you post a screenshot? If Arabic script is too small to read we might have to adjust the default styles. Out of curiosity, though, would Arabic script require RTL input?
I think the stylesheet above would actually decrease the font size not increase it. In any case, adjusting the stylesheet should definitely work; the styles above are just an example.
Tropy will load custom styles from:
<theme> is either 'light' or 'dark' (based on which theme is active).
(To make it easier to experiment with the styles you can also enable developer mode in the preferences and open the dev-tools from the developer menu.)