Themes are one of the formatting options that allows you to change the color, font, and effect of your spreadsheet. Themes are applicable for charts, tables, sparklines, hyperlinks, text, etc in your spreadsheet.
To apply any theme to your spreadsheet:
1. Go to Format > Themes.
2. From the side pane, you can pick any desired theme from the list of options.
To customize the theme of your spreadsheet:
1. Go to Formats > Themes.
2. From the side pane, select any desired theme and click Customize.
3. To change the font, select any font from the Font drop-down.
4. To customize the theme colors, go to Colors, click the drop-down next to the preferred color name (Background2, Text2, Accent 1, etc.), and pick any color.
5. Click More Colors to customize any color of your choice and click OK.
Under Colors, Background2 lets you change the background color of a selected range (for which a theme color is already applied). Text2 represents the text color inside that selected cell range. Accent 1 to Accent 6 are categories of colors in different shades that are applied to tables, charts, etc. Hyperlink lets you apply any desired color to hyperlinks in your spreadsheet.