Try with another version : 6.1.7 to 6.1.8(git) : all other theme uninstall but page are OK.
Don't know what's happen but if i have issue : other user's have issue.

Since all other theme are uninstalled : this confirm 6.X is an unstable version …

only way to fix issue

INSERT INTO `lime_template_configuration` (`id`, `template_name`, `sid`, `gsid`, `uid`, `files_css`, `files_js`, `files_print_css`, `options`, `cssframework_name`, `cssframework_css`, `cssframework_js`, `packages_to_load`, `packages_ltr`, `packages_rtl`) VALUES (1, 'vanilla', NULL, NULL, NULL, '{\"add\":[\"css/base.css\",\"css/theme.css\",\"css/noTablesOnMobile.css\",\"css/custom.css\"]}', '{\"add\":[\"scripts/theme.js\",\"scripts/ajaxify.js\",\"scripts/custom.js\"]}', '{\"add\":[\"css/print_theme.css\"]}', '{\"ajaxmode\":\"off\",\"brandlogo\":\"on\",\"container\":\"on\", \"hideprivacyinfo\": \"off\", \"brandlogofile\":\"themes/survey/vanilla/files/logo.png\",\"font\":\"noto\", \"showpopups\":\"1\", \"showclearall\":\"off\", \"questionhelptextposition\":\"top\"}', 'bootstrap', '{}', '', '{\"add\":[\"pjax\",\"font-noto\",\"moment\"]}', NULL, NULL);

You should ping @p_teichmann about this, I think

I still don't understand (looking at code) how vanilla was disable …

MAYBE : in this case : we need to check if parent survey is OK,if not : disable childs…

I will definitely take a look at it as soon as i can. This might have been caused by and old entries in isStandardAdminTheme which didn't make it in the monday release because i discovered that shortly after as well.
Things i will test and maybe you can confirm if you had active,

1. What admin/survey theme was selected before upgrading?
2. Were there custom admin/survey themes installed before upgrading?

@p_teichmann it's not admin theme : it's SurveyTheme

Maybe (unsure) i set default theme to Vanilla . It's my dev computer : no DB backup …
For this issue : https://bugs.limesurvey.org/view.php?id=18906

I put some potential link

@p_teichmann I leave this to you as I understand you have more context.
Is this still open / valid?

@DenisChenu I couldn't reproduce this issue, can you please provide more detailed steps to reproduce it?

Try with another version : 6.1.7 to 6.1.8(git) : all other theme uninstall but page are OK.
Don't know what's happen but if i have issue : other user's have issue.

https://bugs.limesurvey.org/view.php?id=18969#c76172