'More' widget\icon when using a small screen i.e. a mobile phone

[SimonCrutchley]

When using a small screen e.g. mobile phone, to view apps, instead of the full list of widgets appearing in the header bar, you just get one and then a 'More' icon. Clicking on this opens a grid, with all the other widgets in it. Unfortunately, the default icon is white on clear, so if I want to use a white header, for reasons of branding and accessibility, then it disappears (see attached for green version). The icon itself doesn't appear in the widget list, so I wondered whether there is a way to replace it with an alternative of my own making, as I have done with the other icons that appear in the header.

Thanks

Simon