Toolbar buttons are difficult to see when using a dark gnome theme

[Gooberpatrol66]

[hellosiyan]

Viewnior ships with some custom icons for a few of these, but from your screenshot I can see that the theme uses it's own set of icons.
As this is a theme issue, I would suggest reaching out to the theme's author and ask them for support.

[haarp]

I'm running into the same isse on Xfce4, but seeing bright icons on bright background.

It seems most modern desktop environments employ something called symbolic icons, which are monochrome icons meant to adapt their color to the font color. Viewnior doesn't seem to do this, which is bound to run into frequent problems with various theme/icon packs. Should be easy to solve, I think.

Please re-open this, @hellosiyan Thanks!