Issue #3073053 by pivica, Primsi, unstatu, pooja saraah, smustgrave, Berdir: Theme library override can fail in when you have multiple parent themes

Issue #3309247 by pratik_specbee, neclimdul: Unused $theme_engine variable in ThemeInitialization.php

Issue #3222251 by bbrala, longwave: [November 8, 2021] Replace all isset constructs with the null coalescing operator

(cherry picked from commit 3ab61299)

Issue #3069052 by mikelutz, Berdir, longwave, ravi.shankar, voleger, lauriii, catch, andypost: Properly deprecate the stylesheets-remove key from theme info.yml files

Issue #1507896 by b0unty, tuutti, Jeff Burnz, LewisNyman, tameeshb, realityloop, cbanman, umarzaffer, manumilou, mgifford, mducharme, oakulm, lauriii, alexpott, joelpittet, MaskyS, markcarver, xjm: Allow theme developers to add the default logo filename to the theme's .info.yml

Issue #2914595 by _gradient_: \Drupal\Core\Theme\ThemeInitialization declares $moduleHandler dynamically

Issue #2776975 by joelpittet, dawehner, tim.plunkett, xjm, pfrenssen: March 3, 2017: Convert core to array syntax coding standards for Drupal 8.3.x RC phase

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2497667 by almaudoh, Wim Leers, Cottser, Fabianx, jaxxed: Add libraries-extend to themes' *.info.yml

Issue #2451411 by almaudoh, Shamsher_Alam, lauriii, borisson_, cilefen, davidhernandez, Cottser, Wim Leers, joelpittet: Add libraries-override to themes' *.info.yml

Issue #2499943 by mikey_p, Berdir, catch: system.active_theme.THEMENAME state should not cache theme data

Issue #2497259 by catch, tim.plunkett, alexpott: system_region_list() unnecessarily translates region names

Issue #2389735 by lauriii, iMiksu, davidhernandez, larowlan, loopduplicate, LewisNyman, Wim Leers, dawehner, Cottser: Core and base theme CSS files in libraries override theme CSS files with the same name

git commit -m 'Issue #2377397 by Wim Leers, alexpott: Themes should use libraries, not individual stylesheets'

Issue #2328111 by dawehner, martin107, neclimdul: Replace most instances of the DRUPAL_ROOT constant with the app.root container parameter.