Hi, Anke

Firstly, I would like to sincerely apologize for the delayed response as we have been experiencing an unusually high number of tickets. I am sorry that it has taken longer than usual to respond to your concern and your patience is highly appreciated.

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Seems like the active theme you are using, or another plugin might be overriding the CSS styles of Amelia, that would explain why it is behaving differently in the back-end and in the front page.

Here is custom CSS you can try, add it to the WP page, and this should make that text white:

.amelia-app-booking #amelia-booking-wrap #am-step-booking .am-select-date .am-button-wrapper .el-button span, .amelia-app-booking #amelia-booking-wrap .am-step-booking-catalog .am-select-date .am-button-wrapper .el-button span {
    color: #ffffff;
}

Custom code needs to be applied to the page where the booking form is. Depending on what you're using (Gutenberg blocks, or some page builder), adding the CSS or JS can be done in a few different ways. 

If you need help with adding custom CSS to the page, please take a look at this article.

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Try this, let me know how it goes, and check if it does not affect any other styles on the pages which we don't want.

If that happens, we will assist in finding a better solution.