basicui: rgb color selector css improvements

[ako-3004]

Which UI are you reporting an issue for?

• Basic UI

The problem

RGB Color selector dot is not exactly placed, reason is the css styling

Your suggestion

Change color picker handle css class ".colorpicker__handle" like following:

• remove current margin setting "margin: -12px 0 -12px;" which is also wrong (top and bottom are set here, not top and left)
• add "transform: translate(-50%, -50%);" what makes the px value irrelevant. works on all modern browsers.

you instantly see a correct placement exactly in the middle for rgb 100% or on the outer cicle line for a single color 100%

support for older browsers which is not realy relevant today, but possible:

-moz-transform: translate(-50%, -50%);
-ms-transform: translate(-50%, -50%);
-o-transform: translate(-50%, -50%);
-webkit-transform: translate(-50%, -50%);
transform: translate(-50%, -50%);

[lolodomo]

I just tested and you're right the selector is apparently not well placed.
I am going to test your proposed changes in Firefox/Chrome/Edge.