SSMS 18
C:\Program Files (x86)\Microsoft SQL Server Management Studio 18\Common7\IDE
Once the file is opened in the text editor,
scroll down and find the section of the code under the “Remove Dark theme” heading,
add “//” (without quotation marks) at the beginning of the first line like shown below,
and save the file:
powershell -Command "(gc 'C:\Program Files (x86)\Microsoft SQL Server Management Studio 18\Common7\IDE\ssms.pkgundef') -replace '\[\`$RootKey\`$\\Themes\\{1ded0138-47ce-435e-84ef-9ec1f439b749}\]', '//[`$RootKey`$\Themes\{1ded0138-47ce-435e-84ef-9ec1f439b749}]' | Out-File 'C:\Program Files (x86)\Microsoft SQL Server Management Studio 18\Common7\IDE\ssms.pkgundef'"