[myrroddin]

Weird. While I'm sure there is nothing wrong with your copy, it doesn't seem to work for me. "Blizzard Marble" works fine on line 16, but "White Marble" does not.

My main chunk has the following.

-- register font & background with LibSharedMedia-3-0
LSM:Register("font", "Roman SD", "Interface\\AddOns\\MyrroUI\\Media\\Roman SD.ttf")
LSM:Register("background", "White Marble", "Interface\\AddOns\\MyrroUI\\Media\\white marble.tga")
 
function MyrroUI:OnInitialize()
    -- called in MUI_Installer.lua
    self.installer = self.installer or self:CreateInstaller()
end

And MUI_Installer.lua

function MyrroUI:CreateInstaller()
    -- create Installer window ----------------------
    local width = UIParent:GetWidth() / 2
    local height = UIParent:GetHeight() / 2
    local f = f or CreateFrame("Frame", "MyrroUIInstallerFrame")
    f:SetFrameStrata("MEDIUM")
    f:SetMovable(true)
    f:SetUserPlaced(false)
    f:SetClampedToScreen(true)
    f:SetSize(width, height)
    f:ClearAllPoints()
    f:SetPoint("CENTER", UIParent, "CENTER", 0, 0)
 
    -- frame backdrop ---------------------------
    f:SetBackdrop({
        bgFile = LSM:Fetch("background", "Blizzard Marble"),
        edgeFile = LSM:Fetch("border", "Blizzard Achievement Wood"),
        tile = false, tileSize = 20, edgeSize = 130,
        insets = {left = 0, right = 0, top = 0, bottom = 0}
    })
    
    -- "Myrro UI" -------------------------------
    local fo1 = f:CreateFontString()
    fo1:SetFont(LSM:Fetch("font", "Roman SD"), 24, nil)
    fo1:SetPoint("BOTTOM", f, "TOP", 0, -24)
    fo1:SetText("Myrro UI")
    fo1:SetTextColor(201, 137, 16, 1)
    
    -- version text -----------------------------
    local fo2 = f:CreateFontString()
    fo2:SetFont(LSM:Fetch("font", "Roman SD"), 18, nil)
    fo2:SetPoint("TOP", fo1, "BOTTOM")
    fo2:SetText(version)
    fo2:SetTextColor(168, 168, 168, 1)
    
    -- "Installer" ------------------------------
    local fo3 = f:CreateFontString()
    fo3:SetFont(LSM:Fetch("font", "Roman SD"), 18, nil)
    fo3:SetPoint("TOP", f, "BOTTOM", 0, 36)
    fo3:SetText(L["Installer"])
    fo3:SetTextColor(150, 90, 56, 1)
    
    return f
end