Rmd: change syntax highlight behaviour


I am using rmd and wanted to create a pdf output including the codes.
However, an issue is that the chosen highlight works the opposite way I want: underlines the words before "=", instead the words before "<-".
I used monochrome theme.
My goal would be to highlight the points where new variables are defined, not the input parameters.

I am enclosing the yaml as code, and the output as picture.

title: "A1"
fig_caption: yes
highlight: monochrome

Is there a way to do that?
Thank you for your answer in advance