Touch bar icons not illuminating

Try resetting NVRAM/PRAM http://support.apple.com/kb/ht1379

Even though SMC controls the touchbar, some USB is in play, so this would be NVRAM (or reboot.) notice:

USB Device: Apple T2 Bus

USB Device: Touch Bar Backlight

USB Device: Touch Bar Display

USB Device: Apple T2 Controller

BombusBee wrote:

Thanks for the advice, sadly neither an SMC reset, nor resetting the NVRAM/PRAM resolved this issue. Any other suggestions, or is the backlight failure for the touch bar the thing to follow up?

It is a hardware issue that will require service.

In or out of warranty you can get a free over the counter 'Apple Service Diagnostics' test /assessment

Make an appointment for a "hardware issue"—

https://www.apple.com/retail/geniusbar/

Outside the USA

https://locate.apple.com/country

From iOS device using the app to make a online Genius Bar appointment is easy: 

https://itunes.apple.com/us/app/apple-support/id1130498044?mt=8

(note: due to the temporary closure of Apple Stores and the staggered reopening there may be some delays.

 ref: https://www.apple.com/shop/browse/store/temporary_closures )

 “Mac notebook, contact us and we'll send you a box you can use to ship it to an Apple Repair Center. We'll return your repaired product to you as quickly as possible. Depending on where you get service, you might be able to check the status of your repair online.”  Start a service request https://support.apple.com/mac/repair/service

Call Customer Support  (800) MY–APPLE (800–692–7753) 

or on line  https://getsupport.apple.com/

or call  AppleCare Support at 1-800-APLCARE (800-275-2273)

Get your Mac ready for service - Apple Support   https://support.apple.com/en-us/HT209095

You can try an SMC reset: How to reset the SMC of your Mac - Apple Support

Else, the backlight for the Touch bar has most likely failed and your Mac needs service.

Mac Repair - Official Apple Support