Merge pull request #240 from ckeditor/t/ckeditor5-theme-lark/239
Other: The `.ck-progress-bar` styles should not be under with `.ck-content`. Additionaly removed two unused custom properties: `--ck-color-upload-infinite-background` and `--ck-image-upload-progress-line-width`. Closes https://github.com/ckeditor/ckeditor5-theme-lark/issues/239. Closes https://github.com/ckeditor/ckeditor5/issues/1915.