style: use primary colour for icons and text in header when header colour is 'none'

[jfmcquade]

PR Checklist

• [x] PR title descriptive (can be used in release notes)

Description

Follow up to #2216. As pointed out in this comment, when the header colour is none, the icons and text in the header should appear in the primary app colour.

Git Issues

Closes #

Screenshots/Videos

With deployment config:

config.app_config.APP_HEADER_DEFAULTS.colour = "none";

With deployment config:

config.app_config.APP_HEADER_DEFAULTS.colour = "none";
config.app_config.APP_HEADER_DEFAULTS.variant = "compact";