Hi Wu Wei,
The Emphasized theme overrides default button styles.
After playing with palettes I can conclude the problem you mentioned with the Emphasized theme in combination with the Tinka plugin occurs with certain palettes.
When you use the Emphasized theme with 'working' palettes like Contrast Dark, Contrast Light, Nord, ... the Tinka buttons display as expected.
When using 'non working' palettes like Blanca, Blue, Vanilla, ... the Tinka buttons turn white.
So what is the difference between these set of palettes?
The 'working' palettes all define button background and button foreground.
The 'non working' palettes do NOT define the button background and button foreground.
Adding:
button-background: <<colour background>>
button-foreground: <<colour foreground>>or
button-background: transparent
button-foreground: inheritto the 'non working' palettes makes them 'working'.
Hope this helps,
Ton