Media modal close icon should be consistent with other modals

[celloexpressions]

Many of our newer modals use close and arrow icons with borders and a bold background color on focus. Currently, this includes the theme browser, Customizer, media grid details, and probably others.

The media modal should have a similarly-styled close button, probably without the borders. Now that the close button is auto-focused, it needs a more beautiful styling that is consistent with other auto-focused icons (like the Customizer close icon). Otherwise it looks like a bug (#28969).

Patch also cleans up media grid overrides of the modal styling to make it look like the newer-style close icons.

[celloexpressions]

Use media grid close styling for media modal, key media grid & modal icons to color schemes.

[wonderboymusic]

Fixed in [29287].

[helen]

Reverting [29287], then handling further changes in #29147.

[helen]

[29447]