Dark mode UI updates

[mapk]

Fixes #26479.
Provides a dark mode color for caption placeholders as seen in Gallery and Table blocks.
Adds a dark mode color for the block appender button as seen in the Columns block.

How has this been tested?

Tested locally.

Screenshots

Gutenberg Starter Theme

Twenty Twenty One
I'm not sure why the Columns block appender buttons and the Verse block's placeholder text doesn't show correctly in this theme. Anyone have any ideas?

Types of changes

CSS changes.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: +151 B (0%)

Total Size: 1.2 MB

Filename	Size	Change
build/block-editor/style-rtl.css	11.1 kB	+17 B (0%)
build/block-editor/style.css	11 kB	+18 B (0%)
build/block-library/editor-rtl.css	8.94 kB	+7 B (0%)
build/block-library/editor.css	8.94 kB	+8 B (0%)
build/block-library/theme-rtl.css	791 B	+50 B (6%)	🔍
build/block-library/theme.css	792 B	+51 B (6%)	🔍

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.53 kB	0 B
build/api-fetch/index.js	3.34 kB	0 B
build/autop/index.js	2.73 kB	0 B
build/blob/index.js	664 B	0 B
build/block-directory/index.js	8.58 kB	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-editor/index.js	130 kB	0 B
build/block-library/index.js	145 kB	0 B
build/block-library/style-rtl.css	7.75 kB	0 B
build/block-library/style.css	7.75 kB	0 B
build/block-serialization-default-parser/index.js	1.78 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	47.6 kB	0 B
build/components/index.js	172 kB	0 B
build/components/style-rtl.css	15.3 kB	0 B
build/components/style.css	15.3 kB	0 B
build/compose/index.js	9.62 kB	0 B
build/core-data/index.js	12.1 kB	0 B
build/data-controls/index.js	679 B	0 B
build/data/index.js	8.61 kB	0 B
build/date/index.js	31.8 kB	0 B
build/deprecated/index.js	769 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.42 kB	0 B
build/edit-navigation/index.js	10.7 kB	0 B
build/edit-navigation/style-rtl.css	881 B	0 B
build/edit-navigation/style.css	885 B	0 B
build/edit-post/index.js	306 kB	0 B
build/edit-post/style-rtl.css	6.37 kB	0 B
build/edit-post/style.css	6.35 kB	0 B
build/edit-site/index.js	22.2 kB	0 B
build/edit-site/style-rtl.css	3.79 kB	0 B
build/edit-site/style.css	3.79 kB	0 B
build/edit-widgets/index.js	26.6 kB	0 B
build/edit-widgets/style-rtl.css	3.09 kB	0 B
build/edit-widgets/style.css	3.09 kB	0 B
build/editor/editor-styles-rtl.css	480 B	0 B
build/editor/editor-styles.css	482 B	0 B
build/editor/index.js	42.6 kB	0 B
build/editor/style-rtl.css	3.85 kB	0 B
build/editor/style.css	3.84 kB	0 B
build/element/index.js	4.44 kB	0 B
build/escape-html/index.js	735 B	0 B
build/format-library/index.js	7.47 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	1.74 kB	0 B
build/html-entities/index.js	623 B	0 B
build/i18n/index.js	3.55 kB	0 B
build/is-shallow-equal/index.js	712 B	0 B
build/keyboard-shortcuts/index.js	2.38 kB	0 B
build/keycodes/index.js	1.84 kB	0 B
build/list-reusable-blocks/index.js	3.01 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.11 kB	0 B
build/notices/index.js	1.69 kB	0 B
build/nux/index.js	3.26 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.44 kB	0 B
build/primitives/index.js	1.35 kB	0 B
build/priority-queue/index.js	790 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/reusable-blocks/index.js	3.06 kB	0 B
build/rich-text/index.js	13 kB	0 B
build/server-side-render/index.js	2.6 kB	0 B
build/shortcode/index.js	1.69 kB	0 B
build/token-list/index.js	1.24 kB	0 B
build/url/index.js	4.05 kB	0 B
build/viewport/index.js	1.74 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

_{compressed-size-action}

[aristath]

LGTM 👍

[jasmussen]

Looks good, thank you! Pet peeve: I try to avoid the term "dark mode" for this stuff, as it's technically independant of a dark mode feature that I could see landing in the block editor at some point. The is-dark-theme class is set on themes that register themselves as dark, therefore requiring inverted or contrasty colors.

[bph]

Tested locally with Twenty-twenty-one and Twenty-Twenty. It looks so much better now. 👏

[aristath]

The failing tests have nothing to do with this PR so this is safe to merge 👍