LibreOffice Calc comes with a predefined set of formatting themes that you can apply to your spreadsheets.
It is not possible to add themes to Calc, and they cannot be modified. However, you can modify their styles after you apply them to a spreadsheet.
Before you format a sheet with a theme, you have to apply at least one custom cell style to the cells on the sheet. You can then change the cell formatting by selecting and applying a theme in the Theme Selection dialogue.
To apply a custom cell style to a cell, you can open the Styles window and, in its lower list box, set the Custom Styles view. A list of the existing custom defined cell styles will be displayed. Double click a name from the Styles window to apply this style to the selected cells.
To apply a theme to a spreadsheet:
Click the Choose Themes icon in the Tools bar.
The Theme Selection dialog appears. This dialog lists the available themes for the whole spreadsheet and the Styles window lists the custom styles for specific cells.
In the Theme Selection dialogue, select the theme that you want to apply to the spreadsheet.
As soon as you select another theme in the Theme Selection dialogue, some of the properties of the custom style will be applied to the current spreadsheet. The modifications will be immediately visible in your spreadsheet.