What is the THMX file format?
.thmx is a theme file associated with Microsoft Office 2007 and later. Unlike document templates, .thmx files do not contain any document data (such as header or footer sections) - only settings such as fonts, colors, and visual effects. The same .thmx theme can be applied to Word, Excel, or PowerPoint documents, making it the standard way to share a consistent look and formatting across all three applications.
Internally, a .thmx file is a ZIP archive built on Open Packaging Conventions (OPC), the same container used by .docx, .pptx, and .xlsx. Renaming the file to .zip lets any archive tool inspect its contents. The core entry is theme/theme1.xml, a DrawingML part that defines a color scheme, a font scheme, and a format effects scheme. Optional embedded images or custom fonts increase the file size beyond its typical few kilobytes.
Theme files can be generated in any of the above-mentioned applications. A custom theme can be built using the Design tab's theme editor panel in PowerPoint or Word. The exported theme can then be saved as a .thmx file and reused as a shared style source across documents.
Design templates such as .potx, .dotx, and .xltx each embed a theme internally; .thmx is the standalone, portable form that lets you extract and redistribute that theme on its own.
Security & safety
RISK: LOWA .thmx is styling data (colors, fonts, effects), not a macro or executable, so applying one is low risk - it cannot run code. The only real caveats: a theme may embed custom fonts or a background image, and applying a theme overwrites your document's current colors/fonts (keep a backup if you want to revert). Download themes from reputable sources; the rename-to-zip trick lets you inspect contents before applying.
Format details
in a nutshellFULL NAMEMicrosoft Office Themeaka Office Theme file, Office 2007/2010 theme
DEVELOPERMicrosoftsince 2007 (Office 2007 'Fluent' UI and OOXML)
CATEGORYSettings Files
MIME TYPEapplication/vnd.ms-officetheme
TYPEZIP-based OOXML theme package (Open Packaging Conventions)
STANDARDOpen · royalty-free
MAGIC BYTES · FILE SIGNATURE
OFFSET
00010203
HEX
504B0304
ASCII
PK··
A .thmx is an Open Packaging Conventions (OPC) ZIP, so it begins with the ZIP signature "PK\x03\x04" like .docx/.pptx/.xlsx. Identify a theme by its internal parts: [Content_Types].xml plus a theme/theme1.xml DrawingML <a:theme> element. Renaming to .zip lets any archive tool inspect it.
Programs that open THMX files
Windows3 apps
7-Zip
Open-source Right-click > Open archive to inspect the theme XML/colors without Office (a .thmx is a ZIP). 
Microsoft Office / Microsoft 365 (Word, Excel, PowerPoint)
Paid Open a file, go to Design (PowerPoint/Word) or Page Layout > Themes (Excel) > Browse for Themes, and select the .thmx to apply it.
PowerPoint (Microsoft 365)
Paid Design tab > Themes dropdown > Browse for Themes > pick the .thmx; or right-click a saved theme to apply. 
macOS1 app
Microsoft Office for Mac (Word, Excel, PowerPoint)
Paid Open a document, use the Design/Themes menu and choose Browse for Themes to apply the .thmx.
Web1 app
Office for the web (PowerPoint/Word)
Freemium Built-in themes work online; applying a custom .thmx is limited - use the desktop apps for full theme browsing. Technical details
deep spec| Container format | ZIP archive (Open Packaging Conventions / OOXML OPC); same envelope as .docx, .pptx and .xlsx |
| Core XML schema | DrawingML per ECMA-376 / ISO/IEC 29500; root element a:theme inside theme/theme1.xml |
| Magic bytes | 50 4B 03 04 ("PK") at offset 0 - standard ZIP local-file-header signature |
| MIME type | application/vnd.ms-officetheme |
| Alternate MIME type | application/vnd.openxmlformats-officedocument.themeoverride+xml (for theme override parts embedded inside OOXML documents) |
| Encoding | Binary ZIP envelope wrapping UTF-8 XML entries |
| Compression | DEFLATE applied per ZIP entry |
| Color scheme | 12 named color slots (dk1, dk2, lt1, lt2, accent1-accent6, hlink, folHlink) defined in the a:clrScheme element |
| Font scheme | Major (heading) and minor (body) font pair defined in the a:fontScheme element |
| Effects scheme | Three preset intensity levels - subtle, moderate, and intense - defined in the a:fmtScheme element |
| Content Types map | [Content_Types].xml at ZIP root maps each OPC part to its content type per the OPC specification |
| Typical file size | 5 KB - 2 MB; grows when background images or embedded fonts are included |
| Integrity check | Per-entry CRC-32 checksum (ZIP); no document-level digital signature by default |
| Scope of application | Applies uniformly to Word, Excel, and PowerPoint; the same DrawingML theme is embedded inside .potx, .dotx, and .xltx design templates |
| Released | 2007 (Office 2007 'Fluent' UI and OOXML) |
| Latest version | Office Open XML theme (ECMA-376 / ISO/IEC 29500 DrawingML theme part); current Microsoft 365 |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
THMX conversions
Possible
.THMX .ZIPGUIDE
Yes - rename the file from .thmx to .zip; no conversion software is needed.
How to convert
.THMX .XMLGUIDE
Yes - rename the .thmx to .zip, open it with 7-Zip or Windows Explorer, and extract theme/theme1.xml.
How to convert
.THMX .PPTXGUIDE
Yes -- open any PPTX in PowerPoint, go to Design > Browse for Themes, pick the .thmx, and save. The theme is…
How to convert
Not possible
Community Q&A
asked by users
T
Frequently asked questions
What is a .thmx file?
It's a Microsoft Office theme - a reusable set of colors, fonts and effects shared by Word, Excel and PowerPoint. It restyles a document; it is not a document or template itself.
How do I apply a .thmx theme?
In PowerPoint or Word go to the Design tab > Themes > Browse for Themes; in Excel use Page Layout > Themes > Browse for Themes. Select the .thmx and it restyles the file.
What's the difference between a .thmx theme and a .potx/.dotx template?
A theme is just styling (colors/fonts/effects). A template (.potx, .dotx, .xltx) contains starter content AND a theme. You apply a theme to any existing document; you start a new file from a template.
Can I open a .thmx without Microsoft Office?
You can inspect it but not apply it: a .thmx is a ZIP package, so rename it to .zip and open with 7-Zip to read theme1.xml. Actually applying the theme requires an Office app.
Where are my Office themes stored?
Saved custom themes go to your Document Themes folder (on Windows, under %APPDATA%\Microsoft\Templates\Document Themes), which is why they show up in the Themes gallery.
Will applying a theme change my whole document?
Yes - it replaces the document's theme colors, theme fonts and effect styles globally. Content stays, but anything using theme colors/fonts updates. Keep a copy if you might revert.
References
1ECMA-376 Office Open XML File Formats (DrawingML theme)ecma-international.org
2Microsoft Learn - [MS-OI29500] Office Implementation Information for OOXMLlearn.microsoft.com
