[dev-v5][Theme] Fix data-theme for light and system values#4766

MarvinKlein1508 wants to merge 2 commits intomicrosoft:dev-v5microsoft/fluentui-blazor:dev-v5from

MarvinKlein1508:fix-theme-from-bodyMarvinKlein1508/fluentui-blazor:fix-theme-from-bodyCopy head branch name to clipboard