hrm, this is really strange because inspecting any of these svg's for the extensions I can see they have the same color/fill applied other tools. I installed TST, Sideberry and Side View and only Side View was unaffected by this. Sideberry even kept its light grey with dark theme. I think I need someone from the extensions team to speak to whether there is some code that lets extension authors override icon color regardless of theme, which is not great in this case since it looks inconsistent.
Bug 1925701 Comment 1 Edit History
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hrm, this is really strange because inspecting any of these svg's for the extensions I can see they have the should have the same color/fill applied by moz-button styles as other icons in the launcher (bookmarks, history, etc). I installed TST, Sideberry and Side View and only Side View was unaffected by this. Sideberry even kept its light grey with dark theme. I think I need someone from the extensions team to speak to whether there is some code that lets extension authors override icon color regardless of theme, which is not great in this case since it looks inconsistent.