standardize overlay style using CP theme for site welcome message #2675 

#2675

913ae9a914df471b7bc1cf0e9cb76e578685950e use cp theme for welcome message overlay, use build array #2675
#### Changes
* defined variable for welcome message menu path
* changed path to use "cp/" prefix (in order to use cp theme)
* use build array to generate message

ff3d87f62ba92313b2f0219a3ef34cdf4ac82ba3 cleanup: comments, refactor, rename #2675
#### Changes
* rename path to 'cp/welcome'
* comment everywhere
* rename variables
* format code
* separate logic from assignment