Relevant sections of my /etc/config/luci with two themes installed (themes section), and bootstrap being selected: (main section, "mediaurlbase"):
OpenWrt SNAPSHOT, r14576-d96d324280
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root@router1:~# cat /etc/config/luci
config core 'main'
option lang 'auto'
option mediaurlbase '/luci-static/bootstrap'
option resourcebase '/luci-static/resources'
option ubuspath '/ubus/'
config internal 'themes'
option Bootstrap '/luci-static/bootstrap'
option Material '/luci-static/material'
...