1. Check if the styles you wrote were loaded properly, you should see them in the dev tools with a strikethrough if they're not applied
2. increase the specificity of the styles, either by giving them a parent so it'd be .parent .class {} or if that's not enough to override the class of CKEditor you can keep increasing the specificity or adding !important, I prefer not using important unless if I must, because you may want to override the override later.

The background colour of the ckeditor5 window is defined using a CSS variable like this:

.ck.ck-editor__main>.ck-editor__editable {
    background: var(--ck-color-base-background);
    border-radius: 0;
}

You can customise the appearance but importing a new CSS file that overwrites these variables. They provide a good guide here: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/deep-dive/ui/theme-customization.html