Design
Icon colors
Set the color of the close icon.
The Icons card on the Design tab sets the color of the close icon.
Set the close icon color
Open the timer and go to the Design tab.
Find the Icons card.
Set Close icon color.
Click Save.
The close icon only appears if Show close button is ticked on the Content tab, which is available on top/bottom bar timers. See Close button.
Picking a color
Match the close icon to your title color. It keeps the design coherent, and it guarantees the contrast works — if the title is readable on your background, the icon will be too.
Icons are small, so low contrast hurts them more than it hurts text. A grey close icon on a mid-grey bar is effectively invisible, and a visitor who can't find the dismiss control just leaves the page instead.