Hi Shardul,
It looks like your changes have only been done in the scss files, but they need to be translated to the .css files as well.
The easiest way to do this is using another program. I use ScoutApp for this, but you can also use Calypso:
https://docs.mendix.com/howto8/front-end/calypso.
It's pretty straightforward, you select a source folder (scss folder) and a target folder (css folder) and press run.
When you save you scss files the changes are now also done in the css and available in Mendix.
Hope this helps!
I see you have a typo in your import. Not sure if it is in this post or in your file as well.
It should be something like this:
@import custom-datagrid.scss
---- OR ----
It also might be because of your modeler version. There were issues in triggering recompilation of styles. They fixed this in version 9.10 and above