Refactor Dockerfile and enhance CSS for improved theme support

- Updated Dockerfile to standardize environment variable syntax and improve readability.
- Added new CSS variables for chart grid color and enhanced dark theme contrast for CPU chart statistics.
- Modified index.html to utilize new CSS variables for better theme adaptability and improved chart rendering.

Dockerfile

@@ -7,7 +7,7 @@ RUN apk update && \
     build-base python3-dev dos2unix
 
 RUN python3 -m venv /venv
-ENV PATH "/venv/bin:$PATH"
+ENV PATH="/venv/bin:$PATH"
 COPY ./requirements.txt /
 RUN --mount=type=cache,target=/root/.cache/pip \
     pip install -r /requirements.txt
@@ -19,11 +19,11 @@ RUN dos2unix /docker-entrypoint.sh && \
 
 FROM alpine:3.21
 
-ENV LANG C.UTF-8
-ENV LC_ALL C.UTF-8
+ENV LANG=C.UTF-8
+ENV LC_ALL=C.UTF-8
 
-ENV PYTHONUNBUFFERED 1
-ENV PATH "/venv/bin:$PATH"
+ENV PYTHONUNBUFFERED=1
+ENV PATH="/venv/bin:$PATH"
 
 RUN apk add --no-cache --update python3 curl