Add support for themes

[W1ll1am12]

• Added a theme variable in the config
• Added applyThemesOnElement function that sets the parameters from the user defined theme to the element
• Set the high temp line color to "--error-color"
• Set the low temp line color to "--success-color"
• Set the Precip bar color to "--info-color"

The reasoning is if no theme is set the color for high temp is red, low temp is green and precip bar is blue.