chore: added white background to reference architectures to make it easier to read in dark mode

[ludwig-mueller]

Description

added white background to reference architectures to make it easier to read in dark mode

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[surajsbharadwaj]

@ludwig-mueller can you use one of the image url for a sample catalog and see if it works on catalog UI and that picture is see completely ?

[surajsbharadwaj]

/run pipeline