It gets back to how one defines "traditional/classic" style versus "historical" style.

The Highlanders of Scotland captures the dress of the 1860s, in the heart of the Victorian era.

The history of Highland Dress exhibits long periods of stability punctuated by short periods of rapid change.

One of those periods of rapid change was the first quarter of the 19th century, after which Highland Dress remained fairly stable until the dawn of the 20th century, at which time it underwent a near-total transformation.

This being the case, the dress which emerged around World War One, which is still with us today, the dress I consider to be "traditional Highland Dress", saw the falling from use of a number of things which were part of Victorian Highland Dress.

A great window into the attitudes concerning "proper" (i.e., fashionable) Highland Dress in the early 20th century is The Kilt: A Manual of Scottish National Dress by Loudon MacQueen Douglas R.R.S.E., F.S.A. Scot. published in 1914.

He uses the term "Morning Dress" to refer to what was variously called "Field Dress", "Outdoor Dress", or "Day Dress" the latter term being the one that won out as the 20th century progressed. It's what a gent would wear to a Highland Games, a daytime wedding, or any other event happening prior to evening.

He says this about that form of dress:

Morning dress is a very simple affair and consists of Balmoral cap, tweed jacket and vest (sic) with plain horn buttons. The jacket and vest should be made of the lapel pattern with plain epaulettes on the shoulders.

The kilt should be made of heavier material that than worn for evening dress, but that is a matter of choice.

The sporran should be either of plain leather, or badger, but in any case be of the small round pattern.

Stockings should be of ordinary hose material, and should not be of tartan.

Picking up on his stockings comments, from around 1900 through the 1950s writers consistently state that daytime hose should be "self-coloured to match tweed jacket and vest" (1936) "plain in a shade to tone with the jacket" (1938) "plain, of a colour to tone with the kilt; tartan stockings are not correct with day dress" (late 1950s).

Throughout this period and well into the 21st century Day hose are being offered in Lovat Blue, Lovat Green, and Fawn, three popular traditional colours for tweed Day jackets.

Interesting that patterned hose, considered a faux pas in Day Dress for over a century, have recently returned to favour.