NXUI: Fix visual and ergonomy issues with the Dark theme

The following issues need to be fixed in the Dark theme

• the Details icon (nuxeo:details icon) is not visible enough (circle and text should use a light color for better contrast)
• the CSV export icon (nuxeo:csv-export icon) is difficult to distinguish (a light version should be used)
• the links in the Export popup should use a light color
• the color of the suggestion in the dropdown to start a Workflow should provide a better contrast
• the color of the text in the "More" panel should be light

PS: I use "light" as opposite to "dark" in the description, I hope the term is correct.