fix: colour of back button in app bar

[ghost]

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#### [](https://app.codesee.io/r/reviews?pr=545&src=https%3A%2F%2Fgithub.com%2FAnalogIO%2Fcoffeecard_app) #### Legend

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:exclamation: No coverage uploaded for pull request base (main@57e5565). Click here to learn what that means.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #545 +/- ## ======================================= Coverage ? 72.87% ======================================= Files ? 130 Lines ? 1567 Branches ? 0 ======================================= Hits ? 1142 Misses ? 425 Partials ? 0 ``` | [Flag](https://app.codecov.io/gh/AnalogIO/coffeecard_app/pull/545/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AnalogIO) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/AnalogIO/coffeecard_app/pull/545/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AnalogIO) | `72.87% <0.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=AnalogIO#carryforward-flags-in-the-pull-request-comment) to find out more.

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