ThemePicker UI polish fixes

- Change the size of theme preview item header to 32dp
- Update the layout of the font preview screen
- Color the separator in the font preview step for custom
themes the same color as the text
- Change the size of color selector chips
- Update the layout of the custom theme steps screens

Bug: 135943112
Change-Id: Ib9cbfe16db61d21d7245ab3f309491b2607bfb3b