Open the spreadsheet theme
In Google Sheets on a computer, choose Format > Theme. Google documents selecting an available theme or choosing Customize for a custom theme. Make the change in the intended spreadsheet so that an unrelated file is not mistaken for the template you want to maintain.
Understand the affected elements
Google lists grid text, charts and pivot tables among the elements whose font and color can follow the theme. Theme choices also affect other colors used by charts and links. A theme is broader than changing the family in one selected cell.
Recognize formatting overrides
Google notes that formatting an individual item overrides the theme. If one heading or chart looks different after a theme change, compare its own formatting rather than assuming that the theme failed everywhere. Keep deliberate exceptions limited and explain their purpose for the next editor.
Keep spreadsheet roles clear
Use a consistent distinction between labels, entered data and calculated results. Compare long headers, dates and decimal values after a family change because different widths can affect fit. For the separate Microsoft workflow, read the Excel default-font guide.
Your questions, answered.
Does a theme change only the cell font?
No. Google describes theme effects on grid text, charts, pivot tables and several associated colors.
Why did one cell keep its old appearance?
Google states that formatting applied to an item overrides the theme. Inspect the item’s own formatting.
Sources & background
Sources supporting this page’s explanations. Practical examples are our editorial suggestions.