BUG: using a list in "selector" for set_table_styles produces wrong rules
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Problem description

Example: I want to apply a style like this:

`df.style.set_table_styles([{'selector': 'th.col_heading,th.blank', 'props': [('display', 'none')]}])`

Note a list (`th.col_heading,th.blank`) as selector. This produces wrong CSS, because UUID of the table is only prepended once to this selector:

    #T_table_UUID th.col_heading, th.blank {
          display: none;
    }

i.e. __all__ `th.blank` elements in all tables on the page are hidden. This is not exactly intuitive, although the workaround is simple: you need to write

`set_table_styles([{'selector': 'th.col_heading', 'props': [('display', 'none')]}, {'selector': 'th.blank', 'props': [('display', 'none')]}])`

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.2.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.6.10-arch1-1
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.1.3
Cython           : None
pytest           : 5.4.1
hypothesis       : None
sphinx           : 3.0.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.14.0
pandas_datareader: None
bs4              : 4.9.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.16
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
