Two ways, both I just tested successfully.
Option #1 - Use a more precise selector:
div.math mjx-container[jax="CHTML"][display="true"] {
display: block;
text-align: left;
}
Option #2 - forcibly override the setting:
mjx-container[jax="CHTML"][display="true"] {
display: block;
text-align: left !important;
}
Overriding is the brute force answer but it becomes rather messy when dealing with many styles since it breaks inheritance.