That is strange indeed, might I suggest something else? I read your question again, and you say "palette" as in color palette? Then it should simply save automatically after you set the color and click outside of the color selection box.

But in odoo14 i used to use the palette to add blocks and to changes things then there are a discard and a save buttons, but it is not the case for odoo15.
In the recent version i use save button to close that palette also, but in this case i don't know how to close it Lol

I did not add any custom lines of CSS, i just started working with website so it is kinda weird that the save button doesn't appear.