Unfortunately it's not possible to disable this feature at the moment from your end. However, the team is working on it to make it manageable. We have already created a feature request: https://jira.atlassian.com/browse/JSD-2257regarding it. Please feel free to add your comments and vote for it to make it more noticeable to our developers.
Please raise a support ticket in https://support.atlassian.com for us to apply the suggested workaround:
Workaround to disable
Enable the following dark feature from https://<instance-url>/SiteDarkFeatures!default.jspa: (in Cloud Atlassian Support will need to do this)
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