Tools > Options > LibreOffice > Personalisation > Select Theme > search for some random theme, select one, click ok.
The status text "Applying theme" shows, but then LibreOffice quickly and reliably crashes. With some themes (Firefox Personas), the application of a different theme works, but it seems, that with most themes/personas, the crash is very reliable.
This happens since LibreOffice versions 5.0.0 through to 5.1.x, and still happens with 5.0.6.
Both in Windows (32-bit) and Linux (Knoppix).
You could get a backtrace of the crash either on Win or Linux:
I believe I got the same bug with 5.3 alpha1 / Windows 7.
I wend to Tools -> Options...; LibreOfficeDev -> Personalization, clicked Own Theme; in Select Firefox Theme dialog I clicked Color category, and selected the first entry (Best Color Swirl).
Got a crash with this crash report:
On second try it was working though, so the reproduction might not be reliable.
*** Bug 114818 has been marked as a duplicate of this bug. ***
*** Bug 114739 has been marked as a duplicate of this bug. ***
The Firefox themes integration has been reworked in https://gerrit.libreoffice.org/#/c/61449/ and this issue was likely solved too. It will be available in 6.2 (or nightly builds).
Fixed. Verified on:
Build ID: 3cccff0bbd17ecd4ce386c9eaea06bfa3d14115b
CPU threads: 4; OS: Linux 4.15; UI render: GL; VCL: gtk3;
TinderBox: Linux-rpm_deb-x86_64@86-TDF, Branch:master, Time: 2018-10-27_16:28:45
Locale: ro-RO (ro_RO.UTF-8); Calc: threaded