feat: rebase app themes to material themes

Objective

• Rebase themes to material themes, by modifying src/main/res/values/styles.xml.
• Other files should be influenced as little as possible

Benefits to F-droid

• Changes like #1962 will be possible
• More beautiful but non-distracting transition/feedback effects will be given for free
• Looks more native to Android
• Looks more modern
• Less components should be written from scratch
• Could do #1965 (closed) in the future
• Enables material theming: https://material.io/design/material-theming/overview.html#material-theming