Add placeholder text to dashboard theme

[FWW]

Currently, placeholder text is not customisable for Category Selector, Number Selector, or Date Selector when added to a header in the dashboard.

As the placeholder text for these items is set to a light grey it means that any foreground colour that is dark or of a similar colour makes the placeholder text almost impossible to read. If someone had a corporate colour matching this placeholder hex, it would be invisible with no ability to customise.

Setting the theme / custom theme text colours all to white do not change the placeholder text.