Replace BLACK, WHITE, and polish functions

Description

Within the MarkOneTheme, we need to replace the BLACK and WHITE constants with true white (#FFFFFF) and true black (#000000), as they currently use colors that are slightly off from those. We also need to replace any colors that use the polish library functions lighten and darken with hex codes. These changes will make our theme more straightforward, so it's easier to determine accessible color combinations moving forward.

The hex code equivalents for the lighten and darken functions are approximately as follows (worth checking the styleguide as these updates are made, to see if the hex code is actually equivalent to the previous function output):

Acceptance Criteria

• the BLACK and WHITE constants use #000000 and #FFFFFF respectively instead of the current colors
• the 7 polish functions are replaced with hex codes that approximately match the current colors

Related Issues

dev-team-notes #29

Locate the MarkOneTheme definition and review the styleguide before changing its color values. Replace BLACK and WHITE with #000000 and #FFFFFF, convert the seven polish lighten/darken uses to approximately equivalent hex codes, and confirm the resulting theme colors match the styleguide.