All my cells become black automatically

Hello Xyndrome​,

This usually occurs when a fill color has been applied to the entire worksheet rather than an Office theme issue. Office themes change the ribbon appearance but they do not turn worksheet cells black. Select the entire sheet by pressing Ctrl + A or by clicking the small triangle at the top left corner between the row numbers and column letters. Go to the Home tab, click the Fill Color paint bucket, and choose No Fill. If the color remains, select the entire sheet again, go to Home, click Clear in the Editing group, and choose Clear Formats. You should also check whether a conditional formatting rule is applying the black fill by selecting the sheet, going to Home, Conditional Formatting, Manage Rules, and removing any rule that sets a black background. In most cases selecting the entire worksheet and setting Fill Color to No Fill resolves the issue immediately.