Solarized theme

**Is your feature request related to a problem? Please describe.**
No, low priority feature request.

**Describe the solution you'd like**
A theme based on Solarized colors.

**Describe alternatives you've considered**
Using other theme. As a light user, Sunset/Outrun as they are the only themes without white background.

**Additional context**
Solarized color could be an alternative to have themes with less contrast.

For example :
Light
--app-primary: #b58900;
--app-background: #fdf6e3;
--app-inset-item-background: #eee8d5;
--app-tab-bar-background: #eee8d5;

Dark
--app-primary: #b58900;
--app-background: #002b36;
--app-inset-item-background: #073642;
--app-tab-bar-background: #073642;

The issue does not name files, so start by finding where theme tokens are declared and where selectable themes are defined, using the provided variables (`--app-primary`, `--app-background`, `--app-inset-item-background`, `--app-tab-bar-background`) as anchors. Add a new Solarized light and dark theme entry with the exact requested colors in the same place as existing themes. Validate by running/building the app and checking theme switching in the UI; done means both palettes are selectable and match the supplied colors.

Scritto dal modello di indicizzazione a partire dal testo della issue.