Thank you that worked. I added the code in /theme/variable.scss.
Is there any chance you could tell me how to manipulate the colour of the close button as well?
That is to say turn the close button text to red as well?
At the moment the toast button seems to be have the following classes:
<ion-button ion-activable="" class="toast-button button button-md button-clear button-clear-md ion-color-light hydrated" fill="clear">OK</ion-button>
Where would I specify that I would like instead of the ion-color-light style class to use the ion-style-dark?