Recently, the following note released:
'2249741 - UR stylesheet requests return with 404 response in the portal'
This note relevant only for 731 sp16/740 sp11 and in case of using LESS theme structure(with theme designer tool).
This issue happen in case backend return path of ur stylesheet which contains at least one url parameter.
For example:
ABAP backend return with following request:
'..../com.sap.portal.theming.webdav.themeswebdavlistener/UR/ls/theme_name/standards.css?version=UR.ls:10.30'
In order to fix this you need to apply patches mentioned in above note.