You can have transparent menu buttons or subtitles by including an alpha channel with the images used with spumux. An alpha channel contains values that represents the opacity or translucency of each pixel. Indexed images usually have a 1bit alpha channel. This means each pixel in the image can be either totally transparent or totally opaque. An 8bit alpha channel will have 256 levels, from totally transparent to totally opaque.
The gimp plugin converts all the layers in the gimp file to 4 color indexed images. That is 3 colors and a 1bit transparent channel (which is really just a special color). So this means your menus/subtitles will either be totally opaque (one of the colors) or totally transparent.
The images used by spumux can each have a maximum of 4 colors. Each color can have it's own 8bit alpha value associated with it. That value is taken from the image's alpha channel if it has one. If it doesn't have one, you can specify which color to use as the transparent color in the the config file. This color will then be totally transparent.
To have semi transparent colors, you will have to convert the image to a format that supports an 8bit alpha channel like png. You can use any editor to change the transparency of the image to an 8bit alpha channel. Since I'm using the gimp dvdauthor plugin I'll do it in gimp.
After exporting the images with the plugin, open the image you want to edit. First convert it to 24bit RGB. To get an 8bit alpha channel right-click on the layer and select 'Add Alpha Channel'.
There are several methods to change the transparency of the image. You can change the transparency of the whole layer with the opacity slider in the layer dialogue. If you want to change the opacity of only one color, you can use the eraser tool. Make sure that 'Fade out' and 'Incremental' is not selected in the eraser toolbox properties. Also select the 'Hard edge' option. Set the opacity to whatever level you want. Just make sure that each color in the image has the same transparency everywhere.
Another method is to enter the 'quick mask' mode. Then use the bucket fill tool set at the desired opacity to fill an area. Exit quick mask mode and then cut (Ctrl-X).
There can only be 4 color/transparent-value pairs.
Do not convert the image back to indexed mode, it will lose the semi transparency. Just make sure there are no more than 4 color/opacity pairs. Also take the other images you will use into account (See this page).