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

Issue #3343198 by andy-blum, murilohp, smustgrave, safetypin: Improve documentation of hook_theme_suggestions_HOOK_alter()

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

Issue #2659940 by almaudoh, alexpott, phenaproxima, Jo Fitzgerald, markcarver, oriol_e9g, dawehner, dsnopek, jibran, larowlan: Extension System, Part III: ThemeExtensionList and ThemeEngineExtensionList

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 #2575735 by almaudoh: LibraryDiscoveryCollector::reset() does not properly reset its $cid, resulting in loading wrong library assets if the active theme changes

Issue #2556785 by Antti J. Salminen, lauriii: Specific preprocess functions doesn't support second level without first level

Issue #939462 by lauriii, Antti J. Salminen, NROTC_Webmaster, mbrett5062, tim.plunkett, tostinni, rteijeiro, dvessel, cilefen, barraponto, theapi, joelpittet, Xano, tuutti, drzraf, Fabianx, markcarver, xjm, catch, Cottser, sun, jenlampton, effulgentsia, davidhernandez, kscheirer, akalata, andypost, hass, rooby, jhodgdon, fubhy, becw, MXT, mlncn, lemunet, chriscalip, mike stewart, PavanL, kevinquillen, gleroux02: Specific preprocess functions for theme hook suggestions are not invoked

Issue #2448847 by dawehner, arlinsandbulte: [regression] Themes unable to implement hook_theme_registry_alter()

Issue #2045927 by herom, martin107, ianthomas_uk, Xano, michaellander, Sutharsan: Replace all drupal_alter() deprecated function calls.

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

Issue #1806478 by Cottser, joelpittet, kevinquillen: Make twig the default engine once all modules templates are converted from .tpl.php to .html.twig.

Issue #1033116 by eojthebrave, c4rl, brianV, tim.plunkett | webchick: Rename template.php to THEMENAME.theme to elimnate ambiguity around 'the template file' vs. 'template files'.

Issue #1430300 by mradcliffe, effulgentsia: Fixed Preprocess functions in an include file fail to get called when the theme implements a suggestion override.