* refactor(embed-core): update theme class names for consistency Standardize theme class names to use 'cal-element-embed-dark' and 'cal-element-embed-light' to improve maintainability and clarity. * refactor(embed-core): replace theme class strings with constants * fix theme not changing --------- Co-authored-by: Tushar Bhatt <95581504+TusharBhatt1@users.noreply.github.com> Co-authored-by: Hariom <hariombalhara@gmail.com>