Issue #3409982 by claudiu.cristea, longwave, alexpott, catch, Ghost of Drupal Past: [D10.2 regression] Theme suggestions cannot alter variables anymore

Issue #3109480 by longwave, catch, bnjmnm, xjm, lauriii, Gábor Hojtsy: Properly deprecate theme functions for Drupal 10

Issue #2902412 by Jo Fitzgerald, andypost, mfernea, andriyun: Fix 'Drupal.Strings.UnnecessaryStringConcat' coding standard

Issue #2935256 by Jo Fitzgerald, voleger, kim.pepper, ibustos, Yogesh Pawar, catch, jibran: Remove all usages of drupal_get_message and drupal_set_message in modules

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 #2226207 by lauriii, mgbellaire, Cottser, m1r1k, Mark Carver, LinL, rachel_norfolk, rteijeiro, skwashd, davidhernandez, euphoric_mv: Make 'template' the default output option for hook_theme().

Issue #1751194 by Cottser, mikl, effulgentsia, benjifisher: Introduce hook_theme_suggestions_HOOK() and hook_theme_suggestions_HOOK_alter().