A while ago, I built a count down timer app based on a designer's free giveaway. It turns out users like it and it is even mentioned on beautifulpixels.
This time I'm building a calculator named Minimalist Calculator, on my own, without any help from a designer. Apple link: Minimalist Calculator, I'll share the screen shots here:


Comments from my friends so far are kind of negative. Something like it looks bare-bone rather than minimalist.
As an app developer, I am looking forward to advice on how I can improve the look and feel of my app, which mainly uses font, color, layout (but specifically little/no images)
Fonts are selectable by user, i.e.:

Any reference work I can learn from? A designer for hire who is reputable in minimalism design you could recommend? Thanks!
UPDATE: About options ...
@OghmaOsiris, I did include an option to disable grid lines:


UPATE 2: Colors
Last night, I added the option to change the colors of:
- keypad button face
- button text, and
- button border
Graphite:

Orange:

It's now on AppStore. Download link: Minimalist Calculator
Please feel free to send me comments.
24is shown after entering(5-1÷5)×5, without touching the equal button. – ohho Jan 17 at 4:17