microcontroller - Doesn't voltage on ground make it not ground anymore?

^{simulate this circuit – Schematic created using CircuitLab}

Beginner here, trying to learn enough to start using a microcontroller.

1. 
   

   If the I/O pins are internally high, wouldn't pressing one of the buttons put voltage on the ground wire, making it no longer ground? Including Vss?

   
   


2. 
   

   I added R1 = 100Ω based on the STM8S003F3 datasheet max "Total current out of VSS ground lines (sink)" = 80 mA, and R2 = 270 Ω based on max "Output current source by any I/Os and control pin" = -20 mA. Am I doin' it rite? Is an additional resistor needed to limit current through (decoupling capacitor) C1?