ck5styles.scss imports the main sass file and prefixes it with .ck-content.
That stylesheet is attached to the page whenever a CKeditor enabled field is displayed, including in admin themes.
When editing in a CKeditor field that's displayed with the bootstrap5 theme everything is displayed correctly according to the theme variables.
However in an admin theme all of the variables are missing. This means all of the colors and other styling set via variables are missing. This is because the bootstrap variables on :root are being prefixed with .ck-content which is invalid and the browser ignores them.
Generate the CSS variables properly. They should either be on :root or .ck-content but prefixing them with .ck-content will require editing upstream bootstrap files which is probably undesirable. Since the CSS variables are prefixed with bs- it's probably safe to just attach them to :root.
Active
4.0
Code
MR fixes issue by generating css variables and attaching them to :root. Comparing the variables from styles.css and ck5styles.css reveals an identical output. The rest of ck5styles.css is unchanged.