While Google Sheets is mostly designed for number-crunching, you can choose to make it more aesthetically pleasing by changing its colors through its Theme feature. To access the Theme feature of Google Sheets, click Format, then click Theme. It appears as the first option in the drop-down box.

A sidebar will appear on the right side of Google Sheets. 

The current theme is displayed on the top portion of the sidebar, alongside a Customize button. You can choose among the existing themes offered by Google Sheets as they are listed in the sidebar in the form of a graphic showing the colors and the font.

If you still want to customize your own, click the Customize button.

A list of options will appear in the sidebar. This includes an option to change the font of your spreadsheet, and options to change the color of the text, the chart background, the hyperlinks, and the accents of your charts. 

You have six fonts to choose from: Arial, Comic Sans MS, Courier New, Georgia, Roboto, and Verdana. 

If you click on the option to change the color of a feature, a palette will pop up where you can choose from 80 colors. 

If none fit the bill, you can add your own custom colors. Click the small circle with a plus sign under the Custom label in the palette. A pop-up box will appear where you can add your custom color. You can choose by shifting the circle in the Hue spectrum and then shifting the circle inside the color gradient box, or by adding the Hex code of your custom color.

When you are finished, Click Done on the top portion of the Themes sidebar.