Well, the immediate problem I see is that you have a fixed palette space within which you want to enlarge your text and images. HTML isn't going to help with that. More likely you should redesign your icons.
For example, for the six that select a mailbox, you could use a solid background color with larger text. Everything else on that palette seems sufficiently large in your screen shot (and I'm squinting with double vision at the moment).
You could do some menu reorganization but it appears you prefer discrete buttons for every action (rather than, say, drop down menus for the horizontal menu or fly-outs from the vertical menu). You could put all of your app activation macros under one drop down in the horizontal palette to eliminate that vertical palette, as an example. But it you want buttons for them that won't fly, I'm guessing.
Then there's the issue (which you've no doubt resolved) of what happens when you move the Mail window (you don't, I suspect). The palettes stay where they were put.
So I suspect you aren't moving or resizing the window and the opaque backgrounds of the palettes overlay the Mail interface. Dan's Sticky Palettes would allow you to move your existing palettes outside the window to let you move the window and have the Mail interface available (and eliminate the need for toggling visibility of your main palettes).
So for the AS macros that copy emails to folders, you could use larger font sizes by expanding the width of the Mail window and eating up the new space with a wider palette. Or just eliminate the two vertical palettes to its left as suggested above. But then you'd see fewer destinations at a time.
Those changes would allow you to enlarge your custom text and images without resorting to an HTML palette or two, I believe. (But if you want to continue this discussion, I have a feeling we should start a new thread.)