Your email client or device is using dark mode.
After creating an email template entirely from Content Studio app and not using any custom CSS style file, you notice that the email received by your email client has incorrect colors and design.
SAP Marketing Cloud
This happens because some email clients, primarily Outlook, change colors by themselves when using the dark mode there. This leads to some changed/unwanted colors.
However, if these email clients wouldn't change the colors, black colored text would no longer be readable. So, this is the usual behavior for solving accessibility issues with theming (e.g. dark mode).
This is the expected behaviour.
dark mode, night mode, smc, email lite, email template , KBA , CEC-MKT-MEM , Content Studio: E-Mails and other messages , Problem