Custom css folder override or add to original folder

I have actually tried to figure this one out but am not sure I have the full answer so am asking if anyone knows for sure.

I know for custom folders that define language, vardefs, … , I can add a custom folder and it only adds new or updates existing entries when I explicitly define the entry in the custom folder

However, if I use the custom folder for script changes, it overrides the original file so I need to copy the original script to its equivalent custom directory and make changes in the custom file

My question is about css files.

I need to add some css definitions in custom/themes/SuiteP/css/Dawn/style.css

.col-xs-12 .col-sm-8.edit-view-field textarea#post_event_comments_c {width:100%;}
.col-xs-12 .col-sm-8.edit-view-field textarea{width:100%;}
.col-xs-12 .col-sm-8.edit-view-field textarea#fund_project_list_c {width:100%;}

When I look at the original css file at themes/SuiteP/css/Dawn/style.css
it contains a LOT of css

Is my custom file adding to the original css file or overriding it?
If overriding, I assume I have to copy the original css file contents to the custom file and then add my new css

Advice is appreciated

Custom CSS is appended to the end.

Your custom file should be just a small file with a few styles, not a full copy of the core version.


Thank you

I thought so but wnated to be sure