Increase logo visibility in contrast themes / high-contrast mode

[thibaudcolas]

Follow-up to #454. The logos of Wagtail users visible on the site homepage don’t display too well in Windows high-contrast mode / contrast themes. This is particularly problematic for the Royal College of Arts logo, which is fully invisible when displayed in a high-contrast "dark" theme.

I think the ideal fix would be to apply similar styles to the ones used for the logos in the site’s dark theme:

wagtail.org/wagtailio/static/sass/components/_logos.scss

Lines 61 to 68 in e8d4514

	.theme-dark & {
	#{$root}__list-item {
	display: flex;
	align-items: center;
	padding: 10px 20px;
	background-color: $color--white;
	border-radius: 3px;
	}

By adding a forced-color-adjust: none alongside the background color, we’d ensure the logos are always displayed with the intended background color. It seems to me like all those styles could be present for all themes rather than forced-colors mode / the dark theme only, so the logos display as consistently as possible.