FE: Use theme color definition for warning icon

[stklcode]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

We already have a color definition warningIcon in our theme which is close to the hard-coded yellow color in the actual template <WarningIcon>. Use it for consistency and allow theming here.

The original theme color yellow[20] changes the icon from #F2C94C to #FFDD57 which is close, but drops the contrast ratio on white background from 1.58:1 to 1.33:1.
Introduce a darker version yellow[30] with #FFD439, so we get at least up to 1.42:1. See screenshots below for comparison.

Stumbled across this during #1108

Is there anything you'd like reviewers to focus on?

AFAIK the warning icon is only used in the header for outdated versions (see screenshot at the bottom of the attached image)

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• No need to
• Manually (please, describe, if necessary)
• Unit checks
• Integration checks
• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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