Fixed with !empty($oThemeConfig->compatibility->version) at https://github.com/LimeSurvey/LimeSurvey/blob/e871bea897585edf28de26d0723bf02d746210f7/application/models/QuestionTheme.php#L893

After the quick fix :

E:\WebSite\limesurvey>php application/commands/console.php updatedb
Update sqlsrv:Server=DMZ-SQL19-ST-1;Database=LimeSurvey with prefix :lime_ from 424 to 491
PHP Error[8]: Array to string conversion
in file E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\db\schema\CDbColumnSchema.php at line 146
#0 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\base\CApplication.php(832): CErrorHandler->handle()
#1 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\db\schema\CDbColumnSchema.php(146): ConsoleApplication->handleError()
#2 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\db\schema\mssql\CMssqlColumnSchema.php(86): CMssqlColumnSchema->typecast()
#3 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\db\schema\CDbCommandBuilder.php(229): CMssqlColumnSchema->typecast()
#4 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\db\ar\CActiveRecord.php(1082): CMssqlCommandBuilder->createInsertCommand()
#5 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\db\ar\CActiveRecord.php(811): QuestionTheme->insert()
#6 E:\WebSite\limesurvey\application\models\QuestionTheme.php(284): QuestionTheme->save()
#7 E:\WebSite\limesurvey\application\helpers\update\updates\Update_425.php(21): QuestionTheme->importManifest()
#8 E:\WebSite\limesurvey\application\helpers\update\DatabaseUpdateBase.php(47): LimeSurvey\Helpers\Update\Update_425->up()
#9 E:\WebSite\limesurvey\application\helpers\update\updatedb_helper.php(88): LimeSurvey\Helpers\Update\Update_425->safeUp()
#10 E:\WebSite\limesurvey\application\commands\UpdateDbCommand.php(46): db_upgrade_all()
#11 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\console\CConsoleCommandRunner.php(71): UpdateDBCommand->run()
#12 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\console\CConsoleApplication.php(92): CConsoleCommandRunner->run()
#13 E:\WebSite\limesurvey\third_party\yiisoft\yii\framework\base\CApplication.php(185): ConsoleApplication->processRequest()
#14 E:\WebSite\limesurvey\application\commands\console.php(77): ConsoleApplication->run()

I delete all theme in upload/theme/question and let it go …

My opinion : Do not try to update automatically question theme, just check if it's compatible , if not : uninstal.

I just checked with an upgrade from 3 to 6 and could not reproduce anymore, so I guess it is fixed.

yes : seems there are a fix https://github.com/LimeSurvey/LimeSurvey/blob/056ef5fc47c95ee8a06f34c7f7db4fe9d7024b78/application/models/QuestionTheme.php#L901

PS : still think than updating config.xml file are a bad idea.

We know than between 3 and 5 : there are no real Question/ Survey theme compatibility …