Indeed as @abraemer suggested CSS is the way to go here.
You could override the display of all mathjax in your current notebook with the following style:
html"""
<style>
mjx-container {
text-align: left !important;
}
</style>
"""
To have something more custom just for handcalcs you could create a custom struct wrapper around the LaTeXString struct that has a custom show method for MIME"text/html" that gives a specific class (as example) to your custom struct when shown within Pluto.
For this you could use as example AbstractPlutoDingetjes.is_inside_pluto(io::IO) which has exactly that as one of its use cases