Use static color for shade header text elements

Since shade header text elements are only shown on the dark QS background, it doesn't make sense to set them to a theme-dependent color token that assumes the background will also change depending on the theme.

Bug: 385086613
Test: manually verified with dark theme and light theme enabled
Flag: EXEMPT bugfix
Change-Id: Ib10d11ffb37525e2ac3ed1dbd8c0d278197015cd