Can you explain to me why, although the color of inactive application element of tasks element panel is white in Inkscape, on panel it actually looks gray?

https://i.imgur.com/Bn5TcBM.png

https://i.imgur.com/Ln9wlMu.png

I also tried changing the background of the entire file to white but this doesn't affect how it looks on panel. Is color of panel important, or the way it is displayed is somehow hard-coded?

My goal is to make inactive application line to be white (to match text color).

Inkscape being a GTK application, did you set the application style to use the overall Plasma Style? System Settings -> Appearance -> Application Style -> Configuring GNOME/GTK Application Style...

It is not about Inkscape look, It is about tasks widget look. About this gray line.
https://i.imgur.com/Tom3oi9.png

This depends on the Plasma color style, I really don't see the connection to Inkscape...
Or did I just misunderstand and you try to change the Plasma color style by editing it with Inkscape?