Fix incorrect font color read from xlsx

[anhoder]

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Fix #3464.

[oleibman]

I believe your solution is correct, and I will merge it after studying it a bit more. I just want to add a little documentation about the problem. The spreadsheet which exposed the problem uses a RichText element which uses indexed colors and supplies its own color palette, which is relatively unusual (which is why this isn't a widespread problem). The code to read the color palette is processed with the styles. The code to read the RichText element had come before the style processing, and therefore used a default color palette. Moving the code which processes the RichText after the styles allows the correct palette to be used.

[oleibman]

Thank you for your contribution.