Auto merge of #19330 - Eijebong:sanitize_color, r=KiChjang

Add a sanitize_value implementation for the color input

I had to change the test a little bit to avoid some failures due to
color and text both having a sanitizedValue which was making the test
use the first assertion instead of the second one in some cases.

The sanitize_value implementation is pretty simple, we iterate over the
content and checks that the content is 7 characters long, that the first
character is a `#` and then that all the following characters are
hexadecimal. If all those requirements are met, we lowercase the
content, otherwise we put `#000000` in it.

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