[TASK] ClickMenu: Set background via CSS instead of JS

Move background-style from JS to CSS. Since other skinning-
extensions might override color-settings in TBE_STYLES we need
to generate CSS dynamically and late in the process inside t3skin.

Since backend-CSS is concatenated/compressed this will be included
in the generated merged CSS-file.

Change-Id: Ib04bdf0be172081eb463bdeb9dbd846f7c32cea3
Resolves: #54286
Releases: 6.2
Reviewed-on: https://review.typo3.org/26092
Reviewed-by: Stefan Neufeind
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel