I have a default workspace which is not cms.
My tool use CSS of ametys and these CSS contains URL without workspace name.
For example:
tool.css :

.x-tab-toolbar-wrap .x-panel .x-panel-header {
	padding: 4px 3px 3px 5px !important;
	background-position: 0 -4px;
	background-image: url('../../plugins/extjs/resources/images/gray/tabs/tab-strip-bg.gif');
}

The resulting URL in my case is: http://localhost:8084/_cms/plugins/extjs/resources/images/gray/tabs/tab-strip-bg.gif
and the resources doesn't exist, then i got an error.

The correct url of this background image will be in this case:
background-image: url('../../../plugins/extjs/resources/images/gray/tabs/tab-strip-bg.gif');