You can specify the background using the ‘background’ column of the Google Spreadsheet — see https://timeline.knightlab.com/docs/using-spreadsheets.html#slide_background
We do not support changing the text color, because we feel that makes configuration needlessly complex. When you set a non-white background, black text is styled with a light white border so that it is offset from any dark background. If you do want to change the text color, you can do this with CSS, although it does make everything somewhat more complicated. More information about customizing Timeline CSS is at https://timeline.knightlab.com/docs/overriding-styles.html
