7 string references to 'block_theme' in Drupal 8

BlockForm::form in core/modules/block/src/BlockForm.php

Gets the actual form array to be built.

ConfigureBlockFormBase::doBuildForm in core/modules/layout_builder/src/Form/ConfigureBlockFormBase.php

Builds the form for the block.

d6_block.yml in core/modules/block/migrations/d6_block.yml

core/modules/block/migrations/d6_block.yml

d6_block_translation.yml in core/modules/content_translation/migrations/d6_block_translation.yml

core/modules/content_translation/migrations/d6_block_translation.yml

d7_block.yml in core/modules/block/migrations/d7_block.yml

core/modules/block/migrations/d7_block.yml

d7_block_translation.yml in core/modules/content_translation/migrations/d7_block_translation.yml

core/modules/content_translation/migrations/d7_block_translation.yml

SystemBrandingBlock::blockForm in core/modules/system/src/Plugin/Block/SystemBrandingBlock.php