Why not make the page number clickable as it is now ? That saves an icon.

If we also move the search functionality to the index page then this saves another icon.

The result would be the current icons

Upon start up. In most cases, you will want to resume reading from where you left. If you want to go to index then it's an extra click on the index button.

If we always ask the user what to do upon startup then the user has to take an extra action (Which is to tap on a button) to go to the last position. He will also have to tap on an icon in order to go to index.

This means that we introduced an extra tap for the common use case just for nothing

Theme support then. A set of icons and colors. The font is to remain unchanged.
I'm not going to ship SVGs with the default theme. I'll just convert them to PNG graphics without losing quality.

I'll try to sit down at some point and do the SVG icons hopefully.