Changed the plotly default theme (#269)

I think the current blueish plotly default theme bits itself quit beadly
with @KM-R s great work.
I propose to set the deault plotly template to either:

- plotly_white
- ggplot2
- seaborn

Simply change the constant i have set will change this.
Heren you can find the catalog of themes:
https://plotly.com/python/templates/#disable-default-theming

I would not suggest to do more than changing the default layout.

src/aki_prj23_transparenzregister/ui/finance_elements.py

@@ -4,11 +4,14 @@ from io import StringIO
 
 import pandas as pd
 import plotly.graph_objs as go
+import plotly.io as pio
 from dash import Input, Output, State, callback, dcc, html
 from sqlalchemy.orm import Session
 
 from aki_prj23_transparenzregister.ui import data_elements
 
+pio.templates.default = "plotly_white"
+
 COLORS = {
     "light": "#edefef",
     "lavender-blush": "#f3e8ee",