@Achim: The reason for this is: The default theme is too dark in Ubuntu! The composeToolbar2 is so dark, that some internal calculations activate the brighttext mode, which activate the (bright) image from the dark theme. That does sound sensible to me. Magic is happening here: https://searchfox.org/comm-central/source/mail/base/content/toolbarIconColor.js#103-129
Bug 1670017 Comment 21 Edit History
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@Achim: The reason for this is: The default theme is too dark in Ubuntu! The composeToolbar2 is so dark, that some internal calculations activate the brighttext mode, which activate the (bright) image from the dark theme. That does sound sensible to me. Magic is happening here: https://searchfox.org/comm-central/source/mail/base/content/toolbarIconColor.js#103-129 I will check if we can do something about this.