Customization: Add instructions for changing search button color

[Samk13]

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[fenekku]

Thanks for this, can be merged after adjustments.

[fenekku]

Interesting that this is still needed. I have a note in my codebase saying that having to use button.variables could be deleted in v11. Maybe it has to do with something else on our side...

In any case, let's move this below the next paragraph with a little connection so as to get:

---start---

You can override any styling variable in your site.variables file. The available styling variables are typically found in the .variables files of the various Invenio modules installed (see note below).

For instance, to change the search button color, create the assets/less/site/elements/button.variables file (notice the use of the elements folder instead of the globals one) and add the following line to it:

@searchButtonColor: /* your desired color */;

---end---

[Samk13]

Change is implemented thanks for review @fenekku
I also explored the issue at InvenioRDM #1642 without success, yet the current approach seems effective. Let's proceed with this solution.