I was recently trying to make AquaMail look like Yahoo, and thought about posting the beautiful Yahoo theme that I ended up with.
Select Material Design light theme from Settings > Look and feel > Color theme
Go back till Material Design light theme gets applied
Now go to Settings > Look and feel > Title bar and icon bar color > Exact
For Yahoo like appearance set the color as 7B0099
Go back till the Title bar and icon bar color gets applied
The above steps should give an appearance very close to Yahoo and I must say it looks very beautiful.
