OK, so the menu button text for subcategories I was referring to is called a "filter" by Roku. And according to the roAppManager it is controlled by FilterBannerActiveColor
So using the technique I was taught above I changed it to my desired color by adding
theme.FilterBannerActiveColor = "#f8c103"
it looks like this now...
Sub initTheme()
app = CreateObject("roAppManager")
theme = CreateObject("roAssociativeArray")
theme.OverhangOffsetSD_X = "72"
theme.OverhangOffsetSD_Y = "31"
theme.OverhangSliceSD = "pkg:/images/Overhang_Background_SD.png"
theme.OverhangLogoSD = "pkg:/images/Overhang_Logo_SD.png"
theme.OverhangOffsetHD_X = "125"
theme.OverhangOffsetHD_Y = "35"
theme.OverhangSliceHD = "pkg:/images/Overhang_Background_HD.png"
theme.OverhangLogoHD = "pkg:/images/Overhang_Logo_HD.png"
theme.ButtonMenuHighlightText = "#f8c103"
theme.FilterBannerActiveColor = "#f8c103"
app.SetTheme(theme)
End Sub
and it works!!!