A browner horror is reference by Alexander hope olin the poem Eloisa to Abelard.

"But o'er the twilight groves and dusky caves, Long-sounding aisles, and intermingled graves, Black Melancholy sits, and round her throws A death-like silence, and a dread repose: Her gloomy presence saddens all the scene, Shades ev'ry flow'r, and darkens ev'ry green, Deepens the murmur of the falling floods, And breathes a browner horror on the woods."

I expect the historians reference is to this poem in order to give the statement some gravitas.

In effect the term browner horror doesn't have any deeper meaning in olde English or new apart from the colour. the shades are darker. Period.

Hence from the poem: dusky, black, gloomy, shades, darkens, deepens....browner.