Autoload custom palettes

[cconard96]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.

Description

Automatically load custom palettes the same way core palettes are so they can be easily swapped based on root element attributes.

Example custom palette SCSS (mycustomtheme.scss):

:root[data-glpi-theme="mycustomtheme"] {
    --tblr-primary-rgb: 142, 197, 71;
    --tblr-secondary: #768363;
    --tblr-secondary-fg: #fcfcfc;
    --tblr-link-color-rgb: 69, 148, 54;
    --glpi-mainmenu-bg: #459436;
    --glpi-mainmenu-fg: #f4f6fa;
}

[cedric-anne]

It is a good solution if we consider that people will only use CSS vars for their custom themes, but if their custom themes contains CSS styles (e.g. body{ color:#a2a2a2; }), then they will be applied to everyone, even for users that selected another theme in their preferences.