Issue #3231416 by mherchel, ankithashetty, Gauravmahlawat, chetanbharambe, davidwbarratt: Olivero should not expect its default menu blocks to always exist

Issue #3182200 by andy-blum, kiran.kadam911, mherchel, tushar_sachdeva: Follow proper BEM syntax within secondary navigation menu

Issue #3111409 by mherchel, proeung, larowlan, alexpott, lauriii, Gábor Hojtsy, andrewmacpherson, catch, xjm, andypost, rahulrasgon, rabbitlair, mrconnerton, poojakural, boulaffasae, jerseycheese, shimpy, Indrajith KB, DuneBL, kiran.kadam911, komalkolekar, keboca, hawkeye.twolf, andrewozone, Maithri Shetty, alexdmccabe, sd9121, Sreenivas Bttv, katannshaw, mgifford, nod_, webchick, mtift, rainbreaw, Berdir, brianperry, thejimbirch, Dom., q0rban, nitesh624, hansa11, himanshu_sindhwani, kostyashupenko, shaktik, bnjmnm, sharma.amitt16, sonam.chaturvedi, steinmb, shaal, ressa, Ramya Balasubramanian, Sebacic, pradeepjha, tanmaykadam, thedrupalkid, vinitk, volkswagenchick, viappidu, vebrovski, Ujval Shah, Webbeh, Yuri, trebormc, msuthars, mrinalini9, bash247, chetanbharambe, ellenoise, anevins, andriyun, CocoM, JayKandari, ambuj_gupta, hussainweb, jhodgdon, Pooja Ganjage, ju.vanderw, jwitkowski79, MaxPah, Lokender Singh2, jponch, KarenS, KarinG, Lal_: Add new Olivero frontend theme to Drupal 9.1 core as beta; later make it stable and the default