Change inputbox horizontal cell border colour

[#1]

Hi all,

I am trying to change the colour of the horizontal cell border in an inputbox with multiple rows. In normal view this the colour is not a problem:

But when using webview, this border becomes darker, which is very annoying:

Does anyone know how to change this border colour?

Thanks!
Koen

[#2]

Anybody? I've been looking around on the internet, and haven't found a solution yet.