Invalid CSS color values for popup_bg_hex and popup_text_hex

Problem/Motivation

Config variables popup_bg_hex or popup_text_hex can be set to 000 (integer) rather than '000' (string). If they are, then CSS like this gets generated:

.eu-cookie-withdraw-tab { color: #0;}

#0 is an invalid CSS color value leading to a validation error.

Proposed resolution

In eu_cookie_compliance.module in the color overrides, instead of checking that popup_bg_hex and popup_text_hex are not empty strings, it should check that they match a suitable regex.