But how much are “lifestyle consumption emissions” compared to total emissions?

Yes, good question. In another Oxfam article from 2015 (where they use the same picture as posted here), they explain the term:

emissions associated with household consumption (which we here term ‘lifestyle consumption emissions’)

(page 6)

The idea seems to be: Look at who creates demand for how much emissions. It does not matter if the emissions caused by your household purchase are created in your own country or another country, they are counted as caused by your household.

Note 1 (page 13) has additional information:

The emissions estimates presented in this paper refer to emissions from individual consumption rather than national production, based on the estimates of national consumption emissions provided by Glen Peters of the Center for International Climate and Environmental Research, Oslo. In total, individual consumption makes up 64% of global emissions in the Peters dataset, with the remaining 36% attributed to government consumption, investments (in infrastructure etc) and international transport.