Split up CRM_Utils_System_Base::theme

Split up this method into the different classes so it's specific per CMS.

added improvement label

MR: https://github.com/civicrm/civicrm-core/pull/25329

added has-pull-request label

added Concept approved triaged labels

mentioned in issue #4125 (closed)

mentioned in commit e9e7a2d5

mentioned in commit 0fec25d1

mentioned in commit 5c84abc3

Merged.

closed