There appears to be no way to change the color of the current line highlight, so it’s always yellow. This fails badly when using light text on a dark background (which I’d like to do in Composition mode), leading to white text on yellow.
Also, in composition mode the line highlight has a little jag after the last letter in a paragraph and occasional artifacts between words or on blank lines. This is most visible right now if the background is black.
I am able to change the highlight colour by right-clicking on the highlighter icon in the toolbar and selecting a new color from the grid. Does that not work for you? This grid is also accessible by right clicking on the highlighter icon in the “Formatting” tab found at File > Options > Editing > Formatting.
The highlighting you’re describing colors behind the selected text.
The highlighting causing problems is the one turned on at: File > Options > Appearance>Main Editor>Highlight Current Line. This causes the entire line containing the insertion point to be yellow. On the Mac I can change it to any color (though I can’t remember if it has a separate color setting for composition mode).