fix slick grid index column background in Fast Templates

[MarcSkovMadsen]

Fixes #2613, I.e. gives the index column the right background color in Fast Templates when using dark theme

Still looks fine in default theme

import panel as pn

pn.extension()

import numpy as np
import holoviews as hv

from holoviews import opts, streams
from holoviews.plotting.links import DataLink

hv.extension('bokeh')
curve = hv.Curve(np.random.randn(10).cumsum()).opts(responsive=True, line_width=6)
table = hv.Table(curve).opts(editable=True)
component=pn.pane.HoloViews(table, height=500, sizing_mode="stretch_both")
pn.template.FastListTemplate(title="Table", main=[component]).servable()