Add option to invert color for gains/losses in Balance Sheet and Trial Balance

[hsingll]

Currently, Fava uses green for unrealized gains and red for unrealized losses in the Balance Sheet and Trial Balance when displaying at market value.

I propose adding an optional setting to invert this color scheme (red for gains, green for losses), as some financial markets and regions use the opposite convention.

For example:

• In Chinese stock markets, red typically represents gains (positive) and green represents losses (negative).

• Some users may simply prefer an alternative color scheme for better readability.

[yagebu]

Seems sensible, a PR adding such an option would be welcome :)