@t900502... You can also do it through Splunk UI provided you have access.
You need to go the apps/local page in Splunk UI by clicking on Blue Bolt icon that appears when you click on Splunk menu from the top left.
You can then select your App and Edit Properties to Upload Asset (in your case the CSS file). Provided you have the access to do the same from UI.
Else request Splunk admin to do the same on your behalf.
... View more