From The Document Foundation Wiki
How to customize the user interface?
Beginning from LibreOffice 4.x, it is possible to customize the user interface (see also: “How to change the look of the LibreOffice buttons?”).
- Download a Firefox theme:
- Firefox themes are available at Mozilla’s Add-ons website: https://addons.mozilla.org/firefox/themes/
- Open the theme you have chosen. Right-click it to open the context menu and select the “Save As” option.
- You can download and save several themes in order to test them and check which of them looks the best when applied to LibreOffice.
- Install the downloaded theme:
- Choose ▸ ▸ ▸ ▸
- Type a theme name (e.g., jellyfish, rainbow, nature) or a portion of a name or a keyword (more results can be found as follows: jelly, fish, yellow, red, blue) in the search box. Then, click Search.
- Pick a theme and confirm the selection with OK
You can also search for and directly download a theme using▸ ▸ ▸ ▸ ▸ .
- Click a keyword button (LibreOffice, for example) below the search box.
- Wait for the search to finish and for the nine thumbnails to populate (if the keyword returned a sufficiently large number of themes).
- Click the thumbnail with the theme you prefer, and then click OK to apply the theme.