feat: add theme variables and toggle

[PERRINNSoftwareTeam]

Summary

• replace global Verdana font with imported Roboto and define theme color variables
• use CSS variables in component templates and add light/dark theme toggle
• ensure main color pairs have WCAG-compliant contrast

Testing

• npm test (fails: AssertionError: Compilation cannot be undefined)
• node -e "function l(r,g,b){const a=[r,g,b].map(v=>{v/=255;return v<=0.03928?v/12.92:((v+0.055)/1.055)**2.4});return a[0]*0.2126+a[1]*0.7152+a[2]*0.0722;} function contrast(rgb1,rgb2){const l1=l(...rgb1);const l2=l(...rgb2);const [b,d]=[l1,l2].sort((a,b)=>b-a);return (b+0.05)/(d+0.05);} console.log(contrast([176,186,192],[34,44,50]).toFixed(2));"
• node -e "function l(r,g,b){const a=[r,g,b].map(v=>{v/=255;return v<=0.03928?v/12.92:((v+0.055)/1.055)**2.4});return a[0]*0.2126+a[1]*0.7152+a[2]*0.0722;} function contrast(rgb1,rgb2){const l1=l(...rgb1);const l2=l(...rgb2);const [b,d]=[l1,l2].sort((a,b)=>b-a);return (b+0.05)/(d+0.05);} console.log(contrast([34,44,50],[255,255,255]).toFixed(2));"

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https://chatgpt.com/codex/tasks/task_e_6897a957b6ec83329baaa8de44e8ce04