What is the purpose of blank color palettes

I’m using LibreOffice version 7.4.6.2 (x64) / LibreOffice Community on Windows 11.

When I click the down arrow in the Fill Color dialog (or Line Color), I’m given a choice of color palettes to choose from. Three of these palettes are blank (no color tiles). I know what the custom palette is for, but what is the purpose of the blank Theme colors palette and blank Document colors palette?

The only references to themes in the Writer Guide and Draw Guide are to themes in the Gallery, not the Theme colors palette.

Colours

I can’t say more about the theme color palette than is described in the link.
The document colors store the colors that are applied in a document. So each document can show different color, depending on the usage.
The User Defined Color Palette collects all colors that are entered there by the user and are available LibreOffice-wide.

FWIW, about document themes:

A quick test with Impress (7.4.6.2) – it seems to work.

After scanning the information in the links you provided, I began to get an idea of how I could set up a theme in Impress. The theme doesn’t carry over to Draw.

The implementation of Theme colors and Document colors seems to vary in different components of LibreOffice.

According to the Help (“Colors”), the Theme colors palette shows the theme colors from the current theme, if the document has any, whereas the Document colors palette shows the colors used in the current document. Theme colors only apply to LibreOffice Calc, where formatting themes exist that can be applied to spreadsheets.

I found various themes listed if I went to Format>Spreadsheet Theme and a Theme Selection dialog opened.

I chose a them there. When I tried to change the text color in a cell, however, the Theme colors palette choice still presented a blank palette.

The Document colors palette showed some color tiles though I don’t know where they came from as the document displayed no colors.

I am not familiar with the themes feature myself. This is recent, and currently implementation appears still limited.