The Budgie theme this is based on uses soft white on dark grey main colours, with alizarin crimson highlight colour. For best results making Budgie match this theme:
1) Open Budgie Desktop Settings. 2) Select the 'Style' category on the left. 3) Set 'Widgets' to Matcha-Dark-Aliz so the general colour scheme matches. 4) Set 'Icons' to Papirus-Dark so your Panel Applets match Firefox's white text and icons. 5) Set 'Built-in theme' to Disabled so your taskbar panel will match Firefox's toolbar background colour.
