Update orange brand color less variable in morpheus theme #18709

If I see this right then @color-orange-brand: #ff9600; isn't used anywhere.

Yet at the same time we're using #f57c00 quite often for variables. In https://github.com/matomo-org/tag-manager/pull/416/files?short_path=2b826b2#diff-a3310714e534480dda01c12cc1fdd9062b60925c446ff3fad5e51b7bec4c9fa0R3 orange was used and I will be suggesting to use #f57c00.

Generally be great though to maybe define <a class='mention' href='https://github.com/color'>@color</a>-orange-brand: #f57c00; so we can use <a class='mention' href='https://github.com/color'>@color</a>-orange-brand maybe instead of #f57c00

Also it will be great if we can add the same in Morpheus