Since there are now more and more customisable items in the GUI, Application Colors
has been changed to a menu-driven control. Go to LibreOffice
>Appearance
(a change of name!) and play with Customization Items menu.
However, I don’t understand your question. By default, a TOC has no border unless you configure in a smart and consistent way the Contents xxx collection of paragraph styles. You perhaps mean the background.
This is a visual hint for auto-generated contents. Unfortunately you can’t remove it for TOC.
New in 25.x, there is a boundary hint around the TOC (once again to show the extent of auto-generated contents). You can hide this boundary with View
> Boundaries
. However, I advocate against. This hint does not print but it also controls page, table, section, … boundary display. Since this gives you valuable information about the structure of your document, it is preferable to keep it.
PS: when asking here, always mention OS name, exact LO version (e.g. 25.2.3.2 instead of 25) and save format.