[Users] Fixing interface colors

[mlist]

Hello,

I am using Claws Mail 4.1.1git48 on Linux and XFCE.

While I was using 3.9.x I didn't have this problem but now - there it
is:

When I use a dark XFCE theme, the focus line in the mail list or in the
folder view appears with white color on blue background. However, using
a light theme (Adwaita) causes Claws Mail to show the focus line with
black color on blue background and it is very very hard to read the
text.

There are other UI elements with bad visibility too.

How can I fix this? Or is it a bug to be reported?