I'm using a bunch of app icons in the Dashboard toolbar, and it looks like the dual color icons don't support ghost button text color, or at least the lensApp icon didn't. I also wanted to make them monochrome to match the text.
To make the app icons monochrome and match the text color, I had to apply custom CSS like this to work in both light and dark mode:
https://github.com/elastic/kibana/blob/master/src/plugins/dashboard/public/application/_dashboard_app.scss#L71-L82
Before
Light theme

Dark theme

After
Light theme

Dark theme

I'm using a bunch of app icons in the Dashboard toolbar, and it looks like the dual color icons don't support
ghostbutton text color, or at least thelensAppicon didn't. I also wanted to make them monochrome to match the text.To make the app icons monochrome and match the text color, I had to apply custom CSS like this to work in both light and dark mode:
https://github.com/elastic/kibana/blob/master/src/plugins/dashboard/public/application/_dashboard_app.scss#L71-L82
Before
Light theme
Dark theme
After
Light theme
Dark theme