* Update styles for LLM usage filter form
Enhanced the appearance of the start and end date fields and the filter button by applying new background and text color classes for better UI consistency.
* Replace filter submit button with DS::Button component
Updated the filter form's submit button to use the DS::Button component with secondary variant and medium size for improved consistency with design system.
* Improve calendar picker icon visibility in dark mode
Added CSS to invert the calendar picker icon for date inputs when in dark mode, ensuring better visibility and consistent user experience.
* Add newline at end of application.css
Appends a newline to the end of application.css to follow POSIX standards and improve compatibility with some tools.
* Move date picker dark mode fix to separate CSS file
Extracted the dark mode calendar picker icon fix from application.css into a new date-picker-dark-mode.css file for better organization and maintainability. Updated application.css to import the new file.
