Clean up golden layout base/dark theme scss

[mofojed]

There is some scss in the golden layout theme which doesn't make sense as part of the scss override. For example: https://github.com/deephaven/web-client-ui/blob/cf1b368742a7700b08a8d2eda43671856913c9c7/packages/golden-layout/scss/goldenlayout-dark-theme.scss#L73 The "dark" theme scss should only have colour specifications; all layout scss should be in the "base" theme instead. Perhaps this is something that will be irrelevant with the themeing work @bmingles is doing.