How can I find the face of the items in the headeline in lsp-mode?

Question

Re-edited the answer for clarity

Typically I would put my cursor over the text, and call M-x describe-face, but I can't do that with the headerline.

In the above screenshot, I want to change the headerline to something contrasting. If I could change the light grey to something darker, it would do the job. So I tried to identify the base faces. So far I found these 3:

1. lsp-headerline-breadcrumb-path-face: underlined in red
2. lsp-headerline-breadcrumb-separator-face: the > separator, underlined in yellow (but invisible because it disappears into the background)
3. lsp-headerline-breadcrumb-symbols-face: underlined in cyan

But I can't figure out the spaces in between. If I change the *-symbols-* and *-path-* faces listed above by adding :background "brightblack", and *-separator-* to just :foreground "black", I can only change the underlined parts (see screenshot below).

I also looked at the results of list-faces-display (without my customisation), there is no light grey background (see screenshot below). So I think something other than these faces is giving the light grey background, if I could change that, I wouldn't need to change them individually or find the face for the intermediary spaces.

Follow-up to NickD's question:

header-line-format is set to:

((t
  (:eval
   (window-parameter nil 'lsp-headerline--string))))

When I try to get the value of lsp-headerline--string with C-h v, I get the message [No match].

Answer 1

Use M-x list-faces-display. Use it on a frame that has the same background as the frame where you see the face you're looking for. Find the face that looks like what you see there. Then M-x customize-face.

If you find more than one face that looks the same, then check each one. Use M-x customize-face to change a face's appearance so you can see which one is the one you're looking for.

What you see can also be the result of a combination of faces that are merged at that location. Look for a face that shows you the light gray background you're looking for.

(M-x describe-face also tells you about a face, but it doesn't let you change it, to see whether that's the face you're looking for.)

Answer 2

Thanks to Drew's hints, I found the relevant face by looking at the whole list from list-faces-display one by one. It was coming from header-line, which inherits from mode-line. But I still don't understand why this is happening, because none of the lsp-headerline-* faces actually inherit from header-line. For example:

Hide [Lsp Headerline Breadcrumb Path Face]:[sample]
   [ State ]: STANDARD.
   Face used for breadcrumb paths on headerline.
   [X] Inherit:
       [INS] [DEL] Face: [link] (sample) font-lock-string-face
       [INS]
   Show All Attributes

Maybe header line is special in some way, and applies two faces.