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 2024 - 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 2024 - C:\Users\"user name"\AppData\Roaming\Microsoft\Templates\Document Themes\ 
Excel 2019 - 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

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" />

Colours

SS - Create New Theme Colors dialog box
When you change the theme colours the icons on the ribbon will change to refect the changes made.

Office 2013 and 2016

Text / Background - Dark 1	RGB(0,0,0)
Text / Background - Light 1	RGB(255,255,255)
Text / Background - Dark 2	RGB(68,84,106)
Text / Background - Light 2	RGB(231,230,230)
Accent 1	RGB(91,155,213)
Accent 2	RGB(237,125,49)
Accent 3	RGB(165,165,165)
Accent 4	RGB(255,192,0)
Accent 5	RGB(68,114,196)
Accent 6	RGB(112,173,71)
Hyperlink	RGB(5,99,193)
Followed Hyperlink	RGB(149,79,114)

Copy Colour Palette

Options > Save > Preserve Visual Appearance of the Workbook
SS

Fonts

You can change the default font and font size from the Options dialog box.

SS - Create New Theme Font dialog

link - answers.microsoft.com/en-us/msoffice/forum/all/how-do-i-set-a-custom-default-font-that-excel-will/f50fe244-eb22-4b8a-a477-af72c8156ac6

Effects

It is not possible to create a custom theme effect from the Page Layout ribbon group