Go to 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: https://wiki.documentfoundation.org/QA/BugReport/Debug_Information https://wiki.documentfoundation.org/How_to_get_a_backtrace_with_WinDbg
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: http://crashreport.libreoffice.org/stats/crash_details/75d0109c-6e71-4d71-8720-7497fe804d41 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: Version: 6.2.0.0.alpha1+ 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