  Pulsation frequencies reveal the interior structures of white dwarf stars,
shedding light on the properties of these compact objects that represent the
final evolutionary stage of most stars. Two-minute cadence photometry from TESS
will record pulsation signatures from bright white dwarfs over the entire sky.
We aim to demonstrate the sensitivity of TESS data to measuring pulsations of
helium-atmosphere white dwarfs in the DBV instability strip, and what
asteroseismic analysis of these measurements can constrain about their stellar
structures. We present a case study of the pulsating DBV WD 0158$-$160 that was
observed as TIC 257459955 with the 2-minute cadence for 20.3 days in TESS
Sector 3. We measure the frequencies of variability of TIC 257459955 with an
iterative periodogram and prewhitening procedure. The measured frequencies are
compared to calculations from two sets of white dwarf models to constrain the
stellar parameters: the fully evolutionary models from LPCODE, and the
structural models from WDEC. We detect and measure the frequencies of nine
pulsation modes and eleven combination frequencies of WD 0158$-$160 to
$\sim0.01 \mu$Hz precision. Most, if not all, of the observed pulsations belong
to an incomplete sequence of dipole ($\ell=1$) modes with a mean period spacing
of $38.1\pm1.0$ s. The global best-fit seismic models from both codes have
effective temperatures that are $\gtrsim3000$ K hotter than archival
spectroscopic values of $24{,}100-25{,}500$ K; however, cooler secondary
solutions are found that are consistent with both the spectroscopic effective
temperature and distance constraints from Gaia astrometry.
