Themes

Accessible from the Page Layout tab
SS
Comes with 20 built-in themes
Themes are shared between Excel, Word and PowerPoint
A theme is a collection of formatting settings that include Colours (12), Fonts (heading + body) and Effects (line + fill)
PowerPoint has an extra feature called background style customisations

.thmx

Default Theme

The default theme is Office
You can see the name of the theme assigned to a workbook by hovering over the Colors drop-down.
SS

Excel 365 - C:\Program Files\Microsoft Office\Document Themes 12\Theme Colors 
Excel 2021 - C:\Program Files\Microsoft Office\Document Themes 12\Theme Colors 
Excel 2019 - C:\Program Files\Microsoft Office\Document Themes 12\Theme Colors 
Excel 2016 - C:\Program Files\Microsoft Office\Document Themes 12\Theme Colors 

Creating Themes

It is possible to create your own custom themes.
Page Layout > Themes > Save Current Theme
SS

The dialog box will default to the following folder:

Excel 365 - C:\Users\"user name"\AppData\Roaming\Microsoft\Templates\Document Themes\ 
Excel 2019 - C:\Users\"user name"\AppData\Roaming\Microsoft\Templates\Document Themes\ 
Excel 2016 - C:\Users\"user name"\AppData\Roaming\Microsoft\Templates\Document Themes\ 
Excel 2013 - C:\Users\"user name"\AppData\Roaming\Microsoft\Templates\Document Themes\ 

Double clicking on this type of file will open it in PowerPoint.
The custom themes list is refreshed every time you click on the Themes drop-down

Changing the Colours

You can see the name of the theme assigned to a workbook by hovering over the Colors drop-down
SS

Custom Theme Colours

ActiveWorkbook.Theme.ThemeColorScheme.Load
"C:\program files\microsoft office\Document Themes 12\ ThemeColors\Apex.xml"
"C:\Documents and settings\"username"\Application Data\Microsoft\Templates\Document Themes\

SS - Create New Theme Colors

Text/Background - Dark 2 - Text 2
Text/Background - Light 2 - Background 2

These appear to be swapped round when you display the colour palette ?
Text1 = Text/Background - Dark 1
Background1 = Text/Background - Light 1

Options > Save Tab
SS
Lets you copy a colour palette between workbooks

Access Denied

You do not have enough privileges to complete this operation
xl - Workbook.xml

<workbookPr defaultThemeVersion="124226" />