Cell highlight color mismatch for tables created in Confluence < 7.5

Table cell highlight colors appear too strong in Viewport for tables highlighted in Confluence versions prior to Confluence 7.5

Steps to reproduce:

1. Create a Viewport with Help Center theme

2. Create a page with a table

3. highlight the cells of the table

4. Open in Viewport

Expected result:
Cell highlight colors match the colors set in Confluence

Actual result:
Cell highlight colors are too strong ie. use pure colors

Workaround:
Apply the highlight again after upgrading to a newer Confluence version 7.5+