THE THEME MAGIC FUNCTION Is not present on my site,

THE THEME MAGIC FUNCTION Is not present on my site, how can I change the colors of my theme please?
Could you help me please ?

ThemeMagic is available in Joomla 3 only, it can't be used in Joomla 4 unfortunately.

saguaros
ok that's a shame, can you tell me how to change the cyan blue color here PLEASE

As I can see that you customized this background color in the file: /templates/uber/css/custom.css

Look for this rule - line 23:

.uber-header.header-5.dark-color {
    background-color: #ade3d6!important;
}

You can change the color there.